What would work well with this group?

[Morithias]

We're running an evil gestalt campaign, you might have seen it filled under "it's war", but I'm still doing some last minute prep.

I got a 3rd party class which gives me 2 extra cohorts (4 total), one which I have currently, and one which I get in 5 levels.

However the DM has ruled that no matter how many cohorts I get, I can only deploy one to battle at a time.

The general party seems to be as follows.

Ranger/monk that uses pistols, good with all forms of combat.

Deathstalker/legacy champion, with some minor cleric casting. Dark changling, so hit and run hide in plain sight, type stuff.

Fear mage. Don't know too much about this guy, but from what I can tell he basically has like 5 fear auras, so if you step near him you need to make a save against like 5 effects.

Frenzy berseker/planar shepherd. Wild shape into demon, demon smash.

Those are my friend's characters.

Here's what I have on my plate to deploy otherwise.

Leadership build that revolves around the one feat, not battle material(Prince)

Artificer who is not to be put into battle (other prince).

Paladin of Tyranny/ancestal speaker.

Deepwood sniper/Disciple of dispater (Boom headshot).

Not sure what exactly we're missing out on, but I have one more character to play around with to my liking.

Level 10, gestalt, and evil campaign. I'm thinking some kind of ninja-maid but I'm not sure how well that would work with the Deathstalker kinda already doing that stuff.

Anything we're missing in terms of power? Don't want to miss out on something we might need.

Edit: Forgot to mention this. Due to it being a cohort of the first Prince the character has to be Lawful Evil.

[Mithril Leaf]

Psionic shaper. Max out ranks in craft poisonmaking an have him mass produce greensickeness/black lotus extract. For when you absolutely NEED to murder an entire legion of dragons. Maybe have the other half of it be factotum if I'm following you correct.

[Morithias]

Psionic shaper. Max out ranks in craft poisonmaking an have him mass produce greensickeness/black lotus extract. For when you absolutely NEED to murder an entire legion of dragons. Maybe have the other half of it be factotum if I'm following you correct.

We are a little short on psionics...and poison would be useful.

"If I'm following you correct." Is something not clear? Tell me, so I can correct it please. I tried to give as much information as I could short of posting the actual builds.

[Biffoniacus_Furiou]

If this character would see battle, a Bard of some sort should work well. Use a Harmonizing weapon to maintain regular Inspire Courage, and he maintains DFI and otherwise just casts crowd controls. For gestalt, maybe Bard//Warlock and spam Blinding Eldritch Chains, though he wouldn't benefit from his own IC with that. Perhaps some sort of Sorcerer/War Weaver//Bard, or even go Warblade/War Chanter//Bard. Plan on going into Sublime Chord at 11 and advance it with prestige classes.

For an out of combat support character, I'll agree that a Psion (Shaper) creating poisons would be the way to go. Factotum with it would add tons of skill utility to the group, plus all the other stuff he'd get.

If the new character can tag along with the party but not participate in combat, any sort of Fiend of Possession build would be perfect. Have him possess a character and give them +4 to every ability score, add +6 worth of properties to their primary weapon, and have it use personal-range buffs on anyone you want.