The Anubans having the best air control of the game can be explained by their backstory. A group of egyptians were abducted and transplanted on a moon where they adapted lived and prospered unlike their Earth counterparts which succumbed to oblivion. The gravity of the moon might have played a role on them being very lightweight on Earth-like settings.
Why the UT99 team in 2003 and UC was formed by Malcolm, Brock and Lauren? Malcolm is the default selectable character, in both the Ladder and the Player Setup screens. Plus, if his team isn't chosen, they replace the team the player faces in the Ladder. Also, he was featured a lot in the console version covers, to the point of bordering into Wolverine Publicity before Malcolm's status as a Breakout Character kicked in. As for Brock, he's the one who challenged Xan in the intro of UT99, and both he and Lauren lead Iron Guard, which the players get to fight against in the team game tutorials. Also, Brock and Lauren are the default selectable characters for the Iron Guard set. And finally, why Lauren? Because she was the default selectable female character.
The lack of certain monsters in the Invasion gametype:
Mercenaries: while back in Unreal they were among the enemy classes. In all reality, they appeared only once in the main game (during the Terraniux levels, and Terraniux is their ship, so you were invading their property) and twice in Return to Na Pali (during the UMS Prometheus and Gala's Peak levels, both of which involved raiding abandoned property). The answer is right there in their name: Mercenaries aren't evil entities per se, they just go wherever there's money or anything valuable for their operations. It's possible they may have even rejected Liandri's offer in order to enter into the Tournament as either a team or an Invasion enemy class.
Titans: they're too big to fit on many arenas, move too slow (fighters can dance many circles around them) and even their stone-throwing attack, though damaging, is unimpressive, and incompatible with the metallic floor of many arenas.
The Skaarj Trooper classes would see their weaponry at a disadvantage against the more advanced weaponry the Tournaments use. Even their shields cannot compare with the full-body-covering (Super) Shield Packs of the Tournament fighters.
Devilfishes and Squids: not all arenas have deep water bodies where they can spawn and move.
Tentacles: Not all arenas have ceilings, many of them are purely outdoors.
Return to Na Pali's Predators and Spinners are quite unimpressive regarding their better counterparts, the Skaarj and the Pupae.
The Izarians from Unreal II: The Awakening require a (very weak) lance in order to deal some pitiful damage and have an awful mobility.
