Over the years using RM and RM variants, I've tried several options, from none to about as many as there are PCs in the group (I'm not including one-shot hirelings such as guides, caravan guards, or similar - just "named NPCs" if you wish).
Given the nature of the system, I'm obviously more comfortable with the lower end of the spectrum, but even then I feel dragged down by the number of dice rolls and table reading. Still, I've ended up with a number of dos and don'ts that tend to work for me
- do have the named NPCs be there for a reason other than filling a technical weakness. Romantic relation, friendship, professional buddies, whatever, but no "this is the healer NPC because no one wanted to play a healer".
- do not have the NPCs be the focus of the plot. This is PC territory. The NPCs might be peripheral to the plot but not central (conversely, NPCs that are central to the plot should not be in the same company as the PCs, at least until the PCs actively attach themselves to them).
- given a relative (to the PCs) expertise ranging from "complete nobody" to "nearly omnipotent" with "same as PCs" in the middle, do try to keep named NPCs in a band between "noticeably weaker but not crippingly so" to "slightly better but not in a field that is central to a PC".
- do not have the PCs provide definitive answers to PC questions and problems, but do have them offer opinions... and do not forget to have them being wrong sometimes. NPCs are not paper soldiers, they are fully fleshed characters with their own demeanor, problems and ideals.
- try to avoid attaching more than one NPC to any given PC specifically (see "relations" above). Players tend to relate to one related NPC just fine, but more than that tends to split attention and stunt self-reliance.
- do NOT force a relationship upon a PC. Talk to the player beforehand or, even better, have a pre-campaign solo play that involves the NPC to see if they both click. If they do and the PC finds it natural to have the NPC tagging along, that's great and your NPC is OK. Otherwise, have the NPC live his own life and use it differently (if the PC did click with the NPC, they will likely cross paths sooner or later anyway).
- do not have the NPC displaying one of a kind powers or abilities. They can have abilities none of the PCs have, but those abilities should be at worst known in the world and at best fairly common for someone with the NPC's background.
I must add that I broke most of those rules at least once. I most cases, I wish I hadn't.
Then there are the system-side aspects (which I also try to use for opposing NPCs and faceless opposition):
- try to avoid rolling dice for NPCs for manoeuver actions, unless the NPC is the focus of the scene and there is a real interest in cheking whether they succeed or not. Otherwise, have them perform "as can be expected".
- when an NPC is put against a PC, try to "reverse" the RM engine and have the PCs make all the rolls. The RM combat and critical tables, for instance, can easily be used by replacing an NPC attack roll with a PC defense roll against a NPC-defined base value (NPC base value - PC modified roll = result on the table). Critical rolls can also be made by players (using 101 - player roll as critical result).
[honestly, I can't figure why this mechanism is not proposed as a standard part of the GM toolset].
- when two (or more) NPCs are put against each other, do not roll. Rather, guesstimate the average outcome and how much time it would take to reach that outcome barring any situational change. Then, run the clock until a) the average outcome plays out, b) PCs intervene, or c) the situation changes. Players do not have HUDs with real-time NPC combat status updates - they only know what they see and hear, and you only need to know precisely how much an NPC is hurt when it's healing time (and even then, only if the PCs provide the healing).
I yet have to implement all those aspects formally (i.e. in writing), but the general rule is "focus on PCs all the time, including when NPCs are involved".
I am also working on a mildly heretical variant where combat tables would be drastically compacted : for each AT/AR, only have
- result threshold for crit-less hit and average number of CH inflicted
- for each crit level, result threshold for that crit level and average number of CH inflicted
The more heretical variant would include a streamlining of the critical tables, which would be read on a single d10 (taken as the "units" die of the d100 attack roll). Ideally, I would find a way to have that variant applied to the RR mechanism as well, with the objective of having a "single roll resolution" for all actions - and all rolls on player side.