What are you looking for in a pair? Are you wanting them to breed and lay eggs? If so, you are better off buying a "mated pair" -- but those are expensive. Mated pairs have regularly laid eggs together and shown that they are in fact mating. If you just want the two of them to get along with each other, two small clowns should be fine. One will eventually turn into a female, and the other will stay a small male. Even if they get along fine, they may never become a mated pair and breed. I had a pair of clowns together for 4 years before they mated and laid eggs for the first time. They paired up, were male and female, but weren't a "mated pair" for 4 years.
Also, where they originated from makes no difference. Sometimes there are slight differences in appearance, but it's usually not noticeable.