Jeff,
A 2.5 perc is probably female... you can't tell them apart, or at least I don't know how, other than size. If you get a small one like 1", it should be okay. If you could get one of those hang on specimen holders like you see in the fish stores that might help you figure out her mood

Just put him in it and see if she goes nuts. You'll know almost immediately if she is going to accept the new one. She will charge.. and if he rolls over and shakes... that's him saying 'Hey baby.. slow your roll.. daddy is just admiring the view' At that point she will either tear his fins off or tolerate him until he can work his pimp daddy magic on her and they will get along just fine.
I've got two mated pairs and they still squabble from time to time, so don't panic, but if she locks his fins in her mouth and proceeds to death roll him, fish him out.
One of the other techniques that people use if they have a spare tank handy, is to pull her out, put him in the target tank for a week and then drop her in. It changes the dominant equation a bit, she will still ultimately establish the head of the pack, but will be more prone to leaving him alone as she gets used to the 'new' surroundings.
Its surprising based on all the horror stories you read on RC etc of the difficulties mating a pair up like that, but I know a lot of people who have done it without a problem so I wouldn't hesitate to try it with a much smaller fish. Just be ready to dig him out if she goes postal on him.