Instead of the usual ERT spawning mechanic, it might be less intrusive on the players to instead have a “Join-as-Forsaken” verb, utilizing the Whiskey Outpost UI for choosing caste.
This would still include the limit as to how many Marines are groundside, but also giving the player free choice as to which caste to play and continuously reminding them that the colony is completely infested and always unsafe.
If it feels like the Xenos are too relentless, have a number that recharges similar to the lesser drone spawning system.