MMizzterBlizzard .... I dont know about Knives... only got my first one about 3 weeks ago, it is about 17cm
He is in with some zebra loaches and so far its been ok but only time will tell so I cant comment there
Clowns and stingrays I have had together for nearly 3 years and no problem but the clowns are all minimum about 11cm.
In fact it is hilarious at times as the clowns surf the loaches
Have tried to video it but they stop the minute i try....
The loaches do not suck on the rays as some fish do but, I guess if the clowns are very small they could become ray food so they must be a decent size