Alien 3 and Requiem show eating.
Alien had it scripted that the Alien raided the food lockers; the novel also has it, but it's conspicuously absent in the film. All films are consistent that they don't appear to need to eat to grow. Requiem, for what it's worth, shows that the domed/ridged distinction is made at birth.
Considering some of the Aliens in the hive are relatively fresh in Aliens, you'd still expect to see some domes even if it's a malnourishment thing.