Hi, in my opinion most dogs settle well with another dog, there may be a few tangles at first but only for a few days,my advice would be to get a female and have her fixed as two males are a wee bit more likely to be a small problem. In the beginning they woule do the "I am the boss bit" but that soon works itself out. I do speak from experience as I bred dogs for most of my life and have handled many resues for my breed.
As you say it has to be a rescue, consider contacting someone in the breed of your choice, almost all breeds in the UK have their own rescue group and would be delighted to help you. You will find their details on the net, if you get stuck I may be able to help although now back in Canada the phone is not an option as easily.
Re the rescue place refunding if not compatible, I would ask up front.
If you do get a dog, walk them together on leashes as often as you can, and feed them in separate rooms in the beginning, also I would remove Charlies favourite toys for a while. It would seem to me that Charlie is well socialised and has doggy etiquette so I don't think you would have much of a problem.
Try hard to find out why the dog need to be re housed, anf if it the allergic thing ask a few more questions as tht is a common reason used to give up a dog and rarely true.
Good luck,
XX Jeannine
