If you pick your speakers right you can mix and match to a point, as long as they are fairly close to each other in efficiency and tonal balance. Of course buying all the same kind makes that easier as they should match each other right then without any tweaking at all. That's why all the AV receivers have the individual volume controls for each surround speaker or everything would be out of balance, especially when you consider that not everyone will place all their speakers exactly the same distance apart all the way around even if they all are a perfect match for each other.
I also built my main speakers, then added a Minimus 7 as a center and some Infinity small 3 ways in the rear. it all sounds pretty good together once I get all the levels balanced right. The Minimus 7 takes the most gain to match the others as it is very small so not as efficient I guess, the fronts and backs match pretty good level wise and sound quality as well. I also got a nice Parts Express 12" powered sub, one of the best sounding ones out there for the least amount of money, great bang for buck for what you get.