Here's my idea for dealing with the aliens getting higher lifeforms too early and being able to mass lifeform spam at a moments notice. Institute a system where before an alien can evolve to a higher lifeform, they need to unlock that lifeform by spending their PRes on it first. As a start, I'd set the unlock value as the current PRes cost for each class, such that it would be
Class - Unlock costs - Evolve costs
Gorge - 10 - 10
Lerk - 30 - 30
Fade - 50 - 50
Onos - 75 - 75
In effect, this would double the time till the first lerk/fade/onos could come out and each alien would have to plan to go that class early on. It would also discourage switching between classes at a moments notice because doing so would be prohibitively expensive. For example, say you unlocked and evolved fade, but your team wants to do a lerk rush. Instead of spending 30 PRes, you'd now have to spend 60 PRes (30 to unlock the lerk and 30 to evolve to lerk), making it still possible to lerk rush, but much more expensive and less likely in practice.