As Far as modules go, there are not many, if any, at this time that i am aware of, Official or not. At least not for RMC/RMX.
Now though i have bought RMC myself, have not yet begun to use t as my statndard for the game. I am an OLD school RM2 fan. From what I have seen, though, RMC is pretty darn near the same as RM2 with minor adjustments. RMX is a stripped down model of RMC.
SO, first I would Suggest buying RMC: Arms Law. Arms Law gives a much more detailed explanation of combat combat tactics and skills. Though it does give examples, it does not address Balance issues.
Balance in melee between melee opponents is trickery becuase your stats and BP expenditures each level mean more than your actual level. To explain this better, let m break it down with an example.
2 fighters with the same stats:
One buys skill in 4 different weapons, but only buys 1 rank per level for each skill.
the Other buys skill in only 2 weapons, but buys 2 ranks each level for each skill.
at the end of 5 levels of expenditure, the first fighter has skill in more weapons, and thus more choices, but the second fighter has 25 points more skill in his weapons than the other.
This becomes More important as one becomes more skillful in tactical use of OB-DB.
I have players that NEVER shift much from their OB to their DB, But they also wonder Why My NPCs tend to last longer in fights Unless the players gang up on the NPC. Puttng a portion of your OB to DB does make you hit less and your hits less damaging, but you are also open to being hit more often becuase you are not defending.
So part of balance is to understand how your players will play.
the Second part is Stats.
How are you building your NPCs? If you build the the same way you allow your players to make characters, then it can be considered fairly close to equal. IF you are assigning stats, then you must consider how close are you reflecting the players stats?
Now I don't know if RMX has an NPC table in it, but there is a Generic NPC table for each class in RMC page 157.
If you use this then you must compare the skills/OBs/DBs to that of your players. Consider the Fighter NPC stats, The fighter at level 1 has an OB of 35 and a DB of 30 (which is typically from a normal shield +20 and quickness +10) You can assume that the OB is derived from a base of 3-4 ranks in the skill = +15-+20 and the other 20-15 of the skill coming from the stats bonuses.
This means they are suggesting that the average fighter has a quickness of 90 and an Average str and agility of 98 each. This is, of course, assuming No background options for Average PCs.
Looking at it from a purely Bonus Persepctive, the fighter needs a Quickness bonus of +10 and a str and agility bonus of 15 to 20 each or at least, when both stats are averaged together, a bonus of 15 to 20.
So compare this o your players stats and bonuses. Does the fighter of your Group, at level 1, have an OB of greater than 35? a DB of greater than 30 (with shield included)?
Does his Quickness bonus exceed +10? His Total str and agility bonuses exceed + 20?
The armor worn can then also be a big factor, what do your players have access to at low levels compared to the NPCs.
in the end, for me at least, I go with even odds to 1.5 (based off of total levels) odds at the beginning levels. after level 5 I move to 1.5 to 2 times the players. I tend to kick that up depending on if i am going for 1 or 2 Really strong opponents or a whole lot of weak opponents. 1 or 2 strong opponents and I might kick it up an additional 0.5 times my players since they will be bale to gang up on the opponents. When using lots of weaker opponents I might go to an extra +1x the players and/or use alot of Missile.
When using the Creatures from the Creatures and treasure books and end to deal with them the same way. Though sometimes I will bump up Hit points for tougher monsters.
One thing to note though is that RM is a deadly combat system and it is designed to have the players Feel that Running away is ALWAYS an option, and even that letting some opponents RUN/GEt away is also an option on the side of Prudence.
I have no qualms about Killing my players characters becuase they refused to back down in either case.
Magic in RM2/RMC is a different case when dealing with one on one fights. A level 1 mage against a level 1 fighter might want to run even if he does have distance to get a spell or 2 off. where as a level 10 fighter might want to consider hiding behind cover against that level 10 magician.
The type of Caster can vary the situation as well. Some caster types are not meant for combat of any kind but are more of a support and guide.
From my perspective, at least in RM2, Magic becomes Very powerful in the higher levels. But the mage, who wears no armour and has fewer Hits, can be taken out alot easier by 1 Good shot of a bow.
hope ths helped.