I agree with Joe. When we brought Casie home, part of the agreement in keeping him was that Cobi approve. Cobi is a bigger beagle or was then. He was 6 when Casie arrived, and Casie was 15 months...a little different. We weren't sure that Cobi could be a big bro as he's pretty much been IT since Chipper passed.
But over the year that Casie has been with us...they've had their spats, they've growled, wrestled and yipped at each other. Casie will bite at Cobi's ears, feet, tail whatever he can get and then get out of the way. But when one is sick or not feeling great, they will guard and protect each other --- to the point of not letting us sit on the floor or couch by the sickie.
They snuggle together when they sleep, and share barks at the birds and cats. Casie is still the rabbit hunter, Cobi thinks it's too much work even to bark at them. So as Joe said, they'll work it out. If it gets too physical where one is actually hurting the other, then it's time to revamp....but it's a new member to the pack, and they'll vie for order....be cool and let them take it on...unless of course it's ruthless. Have fun and good luck, we can't imagine living with just one.