I learned most of my lessons on this topic from action and drama films.
I've found that one of the best ways to introduce comic relief in an otherwise tense campaign is to introduce sometimes hilarious events into combat descriptions. For example, let's say one of your party member's gets a critical success on his attack roll and obliterates an enemy. All it takes to add some comic relief then is to throw something like "The guard gives a confused look as the railgun shell comes barreling towards him" or "A growing wet spot appears on his pants as he watches the fireball come closer" to introduce a moment of lessened tension. I particularly like this kind of technique because it doesn't ever distract or detract from the overall mood of the campaign or setting, since combat tends to be a gruesome and serious subject by its nature (and players sense this).
Another great way that I really like is dropping quick one-liners in a tense situation. For example, in a campaign I ran once, my party acquired a spaceship piloted by an NPC. At one point there was some on-board combat, and the party's heavy weapons expert critically failed, making a massive hole in the hull (luckily, they hadn't yet exited the upper atmosphere). The pilot NPC gave the PC an incredulous look and started a slow clap for him, saying, "Way to go, Jim. Way to go." It was a moment of hilarity amidst an otherwise very grim situation, and again, it only added value to the session and atmosphere.
I think there are two keys to successful comedic relief:
- Don't force it; and,
- Make it quick
If you force it, the players will sense it, and it will just be awkward and break the mood (which is exactly what you don't want). Also, if you extend the comedy for too long, it takes over the scenario and your campaign suddenly becomes a silly comedy of errors rather than a drama or action-adventure or whatever else.
Basically, small quips inserted artfully are the key to success.
I figured I'd try to answer the part of the question that most of the other answers haven't really touched on yet, namely "How do I decide whether a particular piece of tech would be too disruptive?"
One way to approach this question is to ask yourself, "How could the players achieve the same effect using things that are listed in the books?" If all the effects of the tech you're considering could already be achieved by other means (even if they might not be quite as practical), it's unlikely to completely break the setting. It might still have an effect on game balance, of course, but you'll at least have some kind of upper bound on how much difference the new tech could make by comparing the difference between the new and the established ways of doing things.
Taking your portable medkit for synthmorphs as an example, you should ask yourself "How else could the players heal a sick or injured synthmorph?" Maybe they can take him/her/it to a hospital; a hospital isn't portable, but it means that all the players are saving by using the medkit is the time and effort to travel to the nearest one. That could still make a big difference, depending on just how far the closest hospital is, but at least it gives you some idea of how big the difference could be. Or maybe your setting has magic as well as tech, and there's already a healing spell that works on synthmorphs, in which case having the medkit just means the players don't necessarily need to bring a mage with healing skills along.
Of course, when using this method, you should generally try to keep the effects of the new tech fairly conservative compared to what already exists in the game. For example, the medkit probably shouldn't be able to fix anything the hospital couldn't. In fact, its abilities should most likely be strictly inferior by a considerable margin, both for the sake of realism and to offset its convenience. Also, since the players can't really keep running to a hospital every few minutes, there should probably also be a limit on how often the medkit can be used, and/or on how long it takes to do its job. And, of course, if the setting clearly implies that something is generally lethal or disabling to synthmorphs, well, the medkit probably shouldn't be able to fix that.
This conveniently segues into another question you can ask yourself, namely "What effects would the existence of this tech have on established (or planned) elements of the setting?" For instance, try to think of any events in the backstory of the setting where having this tech might've made a difference. If you find yourself thinking "Well, those guys sure wouldn't have lost that battle if they'd had these medkits," and you can't think of any good reason why they shouldn't have had them if they existed, that could be an indication that adding that tech to the setting might at least disrupt the consistency of the backstory, if nothing else.
Also, think not just about what your players could or would do with the tech, but also about what their enemies (and other NPCs) could do with it. You're going to need to think about that sooner or later anyway, unless there's some good reason why only the players should have it, so you might as well think about it before you decide that it really exists.
If you have a suitably devious mind, you should also ask yourself "How could I abuse this tech if I wanted to?" Obviously, that means not restricting yourself to how you or the players think the tech should be used, but just looking at the stats as you've written them and thinking "OK, if I wanted to min-max and exploit the hell out of this thing, and had enough resources to pull it off, what could I do?" OK, so one portable medkit seems pretty harmless; what if you had a hundred of them, and combined them with all the most exploity features already in the game, what could you do then?
In particular, beware of anything without limits. A box that can unfold to twice its size is probably harmless. A box that can keep exponentially unfolding forever could easily break the game.
If you're not feeling so devious, a simpler alternative can be to just ask your players up front what they want to do with the tech. Then, if it sounds reasonable to you, write the specs so that it does that and nothing (too much) more. One advantage of this method is that you can harness your players' creativity in finding ways in which the new tech could be troublesome. (In particular, the kind of people who like to come up with exploits for game mechanics are also typically happy to point out those exploits before the mechanics are made part of the game, just as long as they get the chance to demonstrate their cleverness.) It also, in effect, binds the players into an unspoken promise not to step significantly beyond the limits they themselves set, or at least gives you and excuse to step in and say "Hey, wait a minute, that wasn't the way it was supposed to work!" if they do.
Finally, if you're not sure, don't be afraid of saying "OK, let's try it and see." Playtesting is an essential part of balancing the rules of any game, and there's no reason why you can't do that with your house rules too. Just tell your players that they can have the tech for one session to begin with, but that, if it unbalances the game too much, it's going to go away (or be redesigned) for the next session. In extreme cases, you can even tell your players "OK, that was just way too broken, let's just start over and replay the session without it." As long as the players knew in advance that this was going to be a test session, and as long as they can agree that the test didn't work out the way it was supposed to, they'll understand that.
You're trying to set a tone. This is an abstract quality that will inform and emerge from your game. To shape how it goes, you'll need to keep your desired tone in mind during preparation and play, and identify specifics that will help guide you.
Gritty Drama vs. Lighthearted Adventure
You're used to grit. Let's define some of the qualities that comprise it, so you can see more objectively what creates this atmosphere:
Gritty stories are at least as dark, miserable, and challenging - often more so - than the reality we are used to. We can relate to the adversity, and even take comfort that our own world isn't any worse. Empathizing with the characters' struggles is a central appeal; one which is enhanced by maintaining a sense of realism.
Lighthearted adventure stories are about empowerment and escapism. The best of them also highlight the value of virtue, giving the audience inspiration we can carry into our own lives. Notable qualities:
*Example: A hero is kidnapped by bad guys and locked in a dungeon. After escaping their cell, the hero is now in the bad guys' lair and ready to some serious damage.
How the magic happens
As you can see, this is a style that deliberately uses less realism. This is where the room for comedy comes in. Comedy is how we respond to the absurdity of a situation. In a gritty story, one is often too immersed in the drama to laugh much. In a less realistic story you are just enough removed that it's easier to laugh. Also, the lack of realism fosters more absurd situations, so funny things happen more. Don't worry about forcing comedy. It's a spontaneous quality of the genre.