I need your help! 

As of now! Rex has Lumine with him as the main ship.

Should I give Rex a harem? And if Yes, then who?

Fun Fact: Rex can access Morax's memories and in his memories, he sees Lumine as Guizhong

For Reinhardt, Reinhardt has Elysia

should somehow make Sakura and Kallen a part of his harem or not. 

Reve

well, he has Bianka, Rita, and Rave,