Well, just to theorize again, I'd say the guideline would be that certain level grunts could only have certain Pokemon like Carvanha or Slugma. Maybe the Pokemon evolving could even be a way to go up in the ranks.
Even the admins have limited Pokemon, though, which makes me wonder why they would limit themselves so much. (other than because the developers said so. xP) You'd think at least Maxie and Archie would have a larger array of Pokemon, if not simply to show off that they're team leaders.
I mean, there were Gravelers and Torkoals all over the Magma Base, which made me wonder why none of the members actually owned any.