the hpi and proline rims all have the same diameter 5b, 5t, front and rear. the 5t rear rims are slightly narrower than the 5b rear rims by about 5mm or so. the only thing that would give you trouble are the foams. the 5t foams are a lot taller. if you were really determined, you could cram 5t foams in 5b tires on any rim for the baja. this could be a useful tuning aid as it changes the contact patch and preload/air pressure in your tire.
only 2 things make me worry. sidewall deflection and foam sliding side to side with 5t tires and foams on wider 5b rims. also, puckering the bead, trimming the foam and sidewall life of a 5b rear tire on a 5t rear rim. short of putting rear tires on front rims, as long as you have the right foams, there should be no problem.