i have kept mine with rays for years with know problems
18inch i have never seen one that size lets see some pics
they are very slow growing they grow fairly fast up to about 6-7inch then slow right down
they have lots of little back spots on their body when small but will lose them as they get older
its far to risky trying to keep more than one per tank when small they may be ok but will fight to the death as they get bigger
hears mine