Short Answer
The characters don't know, so there's no need to tell their players.
But why's that?
Your GM principles should give you your answer here:
- Address the characters, not the players.
- Begin and end with the fiction
- Give every monster life
Remember, your principles and agendas are rules just like HP and armor.
If you're telling the players the HP and armor of their enemies then you're breaking all three of these principles. The characters don't and shouldn't know that the Goblin over there has 3HP and 1-armor, because those aren't a part of the fiction, they're just numbers to help us interact with it. Likewise to give your monster life you need to describe it. What's more interesting?
You see a dragon on a pile of gold in the cave ahead, it has 16hp and 5-armor and it's coming towards you. What do you do?
Or
Entering the cave the stink of smoke and roasted flesh greets you and laying comfortably atop a pile of gold is a huge red-scaled dragon, currently devouring a helpless goblin. Finishing his meal casually the dragon gets to his feet before you, that fang-laced grin that only a dragon could achieve wide across his wedge-shaped head. "Foolish Adventurers" he says in a voice that rumbles like thunder "Do you really think you are the first to try and claim my hoard? My hide has turned aside all blades brought forth against it and yours will fare no better." As he speaks you can't help but notice the myriad of shallow, harmless scrapes that mark his inch-thick scales. Smoke curls from his nostrils and his eyes gleam with menace as he takes a step towards you, leaving you feeling like a mouse staring down a cat... A huge, flying, armored, fire breathing cat that enjoys playing with his food. What do you do?
I know which one would have me reconsidering how much I wanted that treasure.
It's important to note here that the 16HP and 5-armor aren't what make a dragon a terrifying opponent, with a bit of luck a pair of level 1 characters could bring him down quite quickly if they just kept making Hack&Slash rolls until he dropped. No, the dragon is powerful because it's a goddamn dragon. A typical sword would need to find a weak spot (perhaps the belly) to even have a chance of piercing its hide at all and to make that attack the character would have to get close enough: avoiding the fiery breath, slashing claws and biting maw on the way.
Interesting Reading:
The important point to remember here is that this decision only affects the multiplier on the encounter to determine how difficult it is. Therefore the points to note are:
- This is a "rule of thumb" calculation in the first place. A "Hard" encounter is a very broad area and (despite what the table with its hard boundary suggests) it merges into "Moderate" at one end and "Deadly" at the other.
- It has no impact on the XP the PCs gain in the encounter.
- It is primarily a decision about encounter "pacing" or how many encounters the PCs have before needing to rest. You have way more direct control over this by e.g. bringing an encounter forward if the PCs are flush or deferring one if they are lean on resources.
Therefore it is primarily a judgement call that does not have huge implications. To illustrate consider the examples that you have given:
CR3 Mummy plus (3) CR 1/2 Swarms of Beetles
XP = 700 + 3 x 100 = 1000
Multiplier: 1 or 2 giving 1000 or 2000
CR3 Goblin plus CR1 Lieutenant plus (5) CR 1/4 Swarms of Beetles
XP = 700 + 200 + 5 x 50 = 1050
Multiplier: 1 or 1.5 or 2.5 giving 1050 or 1575 or 2625.
Against say 4 Level 3 PCs, in both cases the first multiplier rates this as "Hard" and any of the others are "Deadly" - what this says is that these are dangerous encounters; be careful. The low CR creatures will have a significant impact on the fight. So using them in the multiplier is reasonable because they low CR creatures will use up significantly more of the party's resources.
Against say 4 level 6 PCs, they are "Easy" to "Medium"; but the low CR creatures will probably be rendered hors de combat in Round 1 - their only contribution is to distract the PCs for that round and maybe burn a fireball. Not using the multiplier would be reasonable here because said fireball will probably be damaging the main protagonist as well so there is no additional drain on party resources.
CR10 and CR1
XP = 5900 + 200 = 6100
Multiplier: 1 or 1.5 giving 6100 or 9150.
This is a deadly encounter for the parties above with the first guy alone (and a probable TPK for the L3 guys).
For 4 level 10 PCs its "Medium" without the multiplier and "Hard" with it. However, given that the CR1 monster can probably be dropped with a Cantrip (3d10), Single attack or at most a 1st level spell (Sleep) it doesn't really add to the difficulty - he will distract 1 of the 4 PCs for part of 1 round. Ignore it.
Best Answer
You have gnolls working with goblins, kobolds, and ogres.
Does the party perhaps have a human, elf, dwarf, and gnome? Because that’s exactly the same level of variety. I’m serious, it can help to think of things in those terms: the “bad guys” can be just as cooperative as the “good guys.”
There’s nothing wrong with this at all. Best is if there is a good reason that they are working together (shared culture/history, or a warlord from among one of the races press-ganging others, or enemies banding together out of desperation, or...), but ultimately the PCs may never learn the reason; you may have just thrown these together because it seemed fun, and the PCs may never even question it and just kill them all, and then the PCs will never learn (and you’ll never have to decide) why they were together to begin with.
But when you notice something in particular about your encounters (or any other part of the setting), it’s good practice to explain it at least to yourself, to have some idea in mind for why things are that way. This will allow you to keep the world more consistent (maybe these particular races are always found living and working together) and/or help to come up with plot hooks (maybe this is very unusual, which leads you to invent some reason why it’s happening, which becomes a situation that the PCs might uncover as they adventure and it gets more revealed).