5-6 bubbletips won't be a problem in your tank but anything over a pair of any clowns will become problematic. UNLESS you were to get them all sub inch and of the same species. I'd only recommend occelaris or Perculas. But your tank will have issues with waste with anything over 6-8 IMO. If you have a sump you can add live rock to it and that will increase your bio filtration. I've got a 40b with 6 fish in it and soon to be 7 and I do have a lot of corals as well, but I also have a sump with quite a bit of lr in addition to what I've got in my tank. Your tank will dictate by water readings what it will support but I'd think over 6 in your 30 and you're pushing it. Just be ready to rehome fish if you start having issues