If you are worried about aggression, how about get a tomato clown, maroon clown, or sebae clown. they can be some tough cookies.
as far as pairing you can do this 1 of three ways. buy mated pair most expesive, buy two and hope they both arn't female the most dominate will become the female, buy one larger and 1 smaller and the larger one should turn female.
if you go on live aquarium they have mated pairs on their all the time right now the cheapest pair are Ocellaris Clownfish (Pair) for 80 and can go up to like $350 for a pair of snowflake clowns.