I just realized that venerating "heretical" saints or heterodox saints probably isn't a big deal. Some Church fathers that we venerate seemed to hold heterodox views, like apocatastasis, millenarianism, etc.

Why does/did the Roman Church allow Uniate churches (previously Orthodox Churches who came into communion with Rome) to keep their saints? Even 'problematic' ones like St. Photius, St. Mark of Ephesus, St. Gregory Palamas, etc.

Also, if an Orthodox Church, like, say, the Russian Orthodox Church were to come into communion with Rome today, would they have to drop their saints who lived and died after the schism?

I think the answer to this question is that yes, the Eastern Rites keep their Saints and devotions to them. The Roman Church probably doesn't have enough cause against certain Eastern saints anyways because some of them are incorrupt. The Russians up to the 1700s believed to be called "Saint" you HAD to be incorrupt. Even up to the 1900s there were and are incorrupt Eastern Saints. If I had to name at least one from the 1900s, I think his name was "Kalymnos the New". When he was exhumed from his grave on an island in Greece, they opened hus coffin to find him incorrupt and it is said the whole island smelled of roses. He was exhumed because he kept appearing to a nun and to the local bishop telling them to do so.

A couple of years ago, Pope Francis Canonized a fellow from the Romanian Church. But, that story needs some explaination, because not all of tgem were monophysite, and our churches have continually had on and off communion/relations.

http://www.catholicworldreport.com/2015/02/26/st-gregory-of-narek-was-the-new-doctor-of-the-church-a-catholic/