Since I only have 3 players, I added another 3 squad mates with character sheets, personality to flesh out the squad and fill in certain roles that the other players are missing. I did tell the Sergeant player that he can tell them what to do in and outside of combat otherwise I am in control, but made sure to keep it very simple in combat.

However they are starting to steal the light from the players. Both in combat and in skill tests. I see where I went wrong with them.

Because of this, I am planning on having them slowly killed off. One of them has already had his guts spilled open and is on the verge of death, so I guess he can be the first to die.

This could also be a good experience for them. For me as well, as a GM.

Now my question is, have you had GM controlled PCs before? Does it work for you? Is it something that requires a lot of attention so it doesn't get out of control? Any tips for using them?

I don't find it necessary, since the Comrades system is already meant to fill the squad with non-important NPC Guardsmen. They react to things, talk to PCs, offer help and insight... but never make Skill tests or actively participate in combat.

I use GMPCs all the time. The trick is that I restrict them to the specialisms that the players don't want to play. This usually means that I end up bringing in GMPC medics and operators to pick up the slack on the skills front due to the fact that my players tend to be kill happy. I would recommend, however, that you limit it to one and you never actually volunteer they do anything beyond general combat input. Always have the PCs ask for their support. For the most part, I try to keep it to one GMPC whom I can then use as a form of direct input on the game. So if the players start being daft or have no clue how to proceed, the GMPC occasionally pipes up with a suggestion to set the ball rolling.

In my GM Circle of about 12 or so gms, it is considered something an inexperienced gm would do to play as a character.

First of as a GM you have many NPC's to act out, let alone other characters. Pushing your self into the player roll , like you said will steal light away from the player.

It may also mean you really just dnt want to gm.

Second thing is that While people often spout rubbish about needing 10 or so players, my experience is five is really the maximum you should have for only war, and three will be the minimum for a combat focused game, even then Only war I have found is best played with a 60/40 Combat to Social Ratio, Hell in my current campaign it's more like 20/80 because my players try to avoid combat being a "stealthy" unit.

My advice Do not use them, I know many players who will walk right out of a game the moment the Gm does it. It shows Inexperience and for alot of people will show a lack of confidence in your GMing ability. You need to be focusing on the story, if you feel that you want to add more in the way of characters that's what comrades are for. If you feel you need more players, just go get them, there are plenty of ways to do it, Roll20 if you are playing online Reddit if you need some one in person.

There is the other option depending on how experienced your players are, you can turn those GMPC to them to play as 2nd/backup PC's. In our long running DH1st campaign we all have multiple PC that are at different levels of exp, which we can use to fill the roles in the party as we need them.

And then when they die in a horrible fashion it will truly mean something to the players.

i started my campaign with about 7-8 people. over time that has been reduced to 3-4. most of the others gave me their character sheets so i keep them on hand if those people manage to get back in (some couldn't play due to work so on the off hand they have the chance to come back in i have their character sheets.) if we come across a mission where we need more numbers, i tend to pass off the other peoples character sheets to the players still in the game. makes things interesting.

I forgot about this.

I am planning on having the rest of squad slowly killed off as the ork forces on the planet begin to crush the Imperial defenses.

Thanks to those that responded though. I love how friendly and helpful it is here.