If they are tiny, just get them about the same size. They may bicker, but it shouldnt be too bad. But if you are worried about them fighting you can get them a different size. Personally, i would just work on picking the two healthiest fish with a smaller consideration for their size.
And there are lots of fish that can kept with clowns, especially if they are the percs or ocsillars (sp?, these are the two most commonly sold clown species). So its more what you like and what can live in your tank size that will determine tank mates. If you buy some of the more aggressive types like tomatoes, then you might be a bit more limited or you can just add them later.