I'm a big proponent of keeping the bed layer drivers the same. The reason is simple: suppose you have characters walking and talking on the screen? Going from left to right, for example, you want their voices to sound the same. But how about when those characters walk from the front of the room to the back of the room? You want their voices to sound the same in this situation as well. The only way to properly do this is to have a similar set of drivers for the entire bed layer.
Triad and some other companies make smaller matching speakers for their surrounds in both monopole and bipole configurations. I created a special Triad Speaker Matching Chart that helps to select the matching speakers for each Triad Series. Email me at the address below and I will send it to you.