Maxx already gave a good answer for when to use Guard Impact so I'll focus on how to play without it.
This is coming from someone who played semi-seriously for months before starting to incorporate GI in SC4.
The first thing to know is your character's fastest moves. It's often just a straight horizontal hit. After that it's a matter of knowing your opponent's attacks and knowing when there's going to be an opening. There's always going to be some time interval either in between or at the end of a combo where you can get in a quick swing,by which I mean you can safely do attack, not necessarily that it will land unblocked. Even if your counter attack is blocked, at least now you're on the offensive.
I've only played SC5 once so far, but it seems like Just Guard gives you a slight edge where you might be able to get in a quick attack before the opponent has a chance to block in some cases, but it comes down to knowing your opponent's moves and how to react to them.
Also some moves have a higher tendency to supersede the opponent's attack than others. I know in general a vertical hit usually cancels out a horizontal hit. Some moves are also faster than the initially appear. I know personally I've found moves that I previously disregarded as not useful turned out to quite useful as they are faster than their animation would make it appear. I think it's an issue of our mind typically watching the time between the start and end of a swing, rather than the time between the start and the time of impact, which may happen well before the end of a swing.
Finally there's a rule of any pretty much any competitive game that has a meta-game component: Set up expectations, then break them. Repeat a pattern until the opponent figures it out, then break the pattern with something that counters what the opponent's going to do to counter you. For example, lots of characters usually have variations on combos, so there's mid-mid-low, repeat that a couple times, then occasionally throw in mid-mid-mid. They'll go to block the low and get slammed with the mid at the end (bonus points if it's a ring out!). It's like in Starcraft, making your opponent over-commit to anti-air, then coming in with a big ground army instead.
Bigger characters have a longer reach and less power. Smaller characters have shorter reach and more power. The optimal setting is the size of the character who uses that specific fighting style. (i.e. if you create a character with Voldo's style and choose the optimal height, that character would have the same reach and power as Voldo in arcade mode.)
Best Answer
The letter-number code on your player card is your multiplayer rank in Ranked matches (not Player matches or Global Colosseo matches). Every time you play a match on XBox Live or PSN, you gain experience towards your next rank (this is separate from the leveling procedure for offline games, which unlocks you additional creation gear). The amount of experience will vary. It is dependent on your opponent's rank and whether or not you win or lose (and probably some other unidentified factors).
The ranks go from E (worst) to A (best), and 5 (worst) down to 1 (best), with a transition to the next highest letter when you get to the lowest number (e.g. E1 levels up to D5). You start at E5, and there are achievements/trophies for achieving higher ranks.
This is also a good way to estimate the difficulty level of opponents in Quick Battle offline; an estimation of appropriate rank is given for each of these opponents.