If I may make a bit of a vague suggestion, typically speakers from the same series are good tonal matches. Meaning? A smaller model from the same batch as your big 'uns make for good surrounds.
Beyond that, are you going to be running simple 4.0 or going for the big matrices such as 7.1? With the former, you tend to want a more spread out, indirect sound such as dipoles or some type of reflecting speaker. With the latter, direct radiators such as your typical cone drivers work just fine as there are more of them.
- JP