First let me say I've never done usability testing. But from what I've read, the right way to do it would be multiple distinct tests, each with a group of 3-5

First let me say I've never done usability testing. But from what I've read, the right way to do it would be multiple distinct tests, each with a group of 3-5 users, with one group for each major user category. Lots of tests with small user groups is much better than fewer tests with more users.