As the game moves on crabs become weaker and weaker, but they are the better counters to railguns and siege tanks, if they attack one by one they are too easy to pick off they need to attack in a swarm. So the solution is to let the commander build crabs in a bundle, with bulk discount in cost and time. FPS players can join the crab swarm but are just a squishy crab and might get unlucky and picked off, the FPS player should be able to just jump to the next crab in the swarm on death.
It could be like converting into a ghost crab. Ghost crab lasts 5 seconds and in that time you can transfer into a nearby alien unit, that's all you can do. Given the swarming nature of the faction this should help FPS players deal with dying a bit better, given humans have teleports its less of an issue for them, and their vehicles take more shots than aliens on average.