For the 'nids The Dominatrix, ruleswise I'd just make it a cross between a Tervigon & a Tyrannofex that poops Warriors insead of Gaunts.
For Da Orkz, Das Uber Grot. He's a gretchen with the WS_6 BS_3 S_5 T_5 W_3 I_6 A_4 LD_10 He's armed with a Khorn Berserker Chain Ax and wears
CSM Power Armor in addtion all Gretchen in the army my buy 'eavy Armor for +5pts. each and all gretchen may use the WAAGH!, Mob Rule! & Furrious Charge Rules (I base this character off a grot from my friends old Ork army who back in 2nd edition Killed 3 Khorn Bersekers in Close Combat all by himself)
It's not a unit, but I'd add Heavy Flamer & Sniper Rifle to the Special/Heavy Weapons choices for Marine
Tac Squads