We should limit higher life forms because marines have no answer to 3 onos
We can do it by this equation:
Onos = 5 Points
Fade = 3 Points
Gorge = 1 Point
Lerk = 2 Points
For every player that each time can have (server slot limit/2) add 1.8 points (Rounded down). For each additional hive add 3 points. Assuming a normal game of 2 hives with an 8 man team this gives a total of 17 points which is enough for 3 onos at one end. A lot of people might object to this because it limits a players actions and it might cause "life form" hogs. Yes, this will result in the sort of behavior, however that is an unavoidable consequence of a necessary balance. There is simply no way to handle herds of onos for marines, and don't try to argue that exo or any tech will change anything because all alien lifeforms come out 6 minutes in, and unless marines are dominating, that means Onos are coming into play early mid game. Exo, or any other tech powerful enough to counter an onos isn't going to be coming early mid game, it's going to be coming late game.
I've experienced many games lately where 4 or more aliens were onos at one time, and it is an extreme problem. It doesn't take more than 3 or 4 RTs for a bunch of skulks to gather enough pres for Onos by six to seven minutes in, which means that the game is being played on a balanced level beforehand.