If the shelter is telling you there is no aggression between it and smaller dogs go for it. Bring your dog with you and let them meet before bringing the dog home.
Is it a feeder dog? I think you could manage that with a group of piranhas or a nice red tail catfish, but I wouldn't risk it with a gar. have you considered using it as a breeder for smaller food? He seems like it would make a pretty consistent source. Thanks.
I would definitely give him a chance.. under very strict supervision.. usually with the smaller dog being there first.. he will respect it..i do not think dogs realize size... but of course not really knowing about his past I would take no chances until everyone was comfortable with each other.. and you will know when this occurs.. my little pom slept on top of my dobie every night.. I am kind of in the same situation here although opposite size in dogs.. my hubby says I can get another small dog..{I wanted another large puppy} I have the collie and dobie huge size.. so I would have to be very careful until I could deem it safe with a small pomerian .. my dobie does not take to other dogs.. but I have been in this position before.. and it is amazing how it all turns out as long as you do it... carefully..consistently and always on quard..... not an easy task but one worth the work...... he is a beautiful boxer..... good luck.