Well, I don't have Chi's, never had but I got one a few years ago for my son, as a Christmas gift. He got another one the next year. About a year ago, his (ex) wife decided she was going to "get rid" of one she had gotten, who was 6 months old at the time. He felt sorry for it, and took it. He still has it, but the dog HATES me-he will bite me. I have NEVER met a dog that didn't like me! Hurts my feelings. The other two dogs he has hate it, because all he(the dog) does is aggravate them-and he won't let THEM near my son!
I guess it all depends on the dog itself, but the Chi may "bully" your dogs! Chi's are known to be jealous and sometimes protective of their owner. Of course, there are good ones. My son's first two are sweethearts but they hate the "new" one. I am sure all depends on each dog.
It seems to me that is Finn is terrtorial , as you say-it would get worse when he gets older-not better. If the shelter would let you do a home test-that would be the best thing-and bring dog back if it doesn't work out.