User blog comment:Pochichi/Aikatsu Stars Presentation/@comment-29740360-20170308231059/@comment-3487190-20170309042847

>  original characters        just be in a different city

The stories are set in different universes apparently. Four Stars and Venus Ark are the top schools in Stars series. The idol ranking is about the whole industry so you cannot say they'r just in different cities. Similiarly, Drefes series is also set in another universe where idol schools don't exist for boys to attend.

A crossover can happen in a non-canon, it doesn't mean that they really connect. Anyway, Stars series wasn't made to be a direct continuation of the original series, if so they could hint on it in episode 1, no need for a crossover episode.