Can you reroll a Lvl 5 pet with another Lvl 5 pet to change stats?

Yes, but as i've read other places, you should wait until T6 pets unlocks. that way u have a chance to get higher tier pets.

If u got +%coins etc and you fuse, and still get coin stats, that would be a bummer :P  But ofc, it depends how many pets u have now and if you got bad skill on them.

LVL 7 pets and yes