I know it's been a while since you posted this question, but as I was tempted to buy the modus a couple of times in the past, I'm now curious about how you find them compared to the diamonds.
However, regarding using tiny surround speakers, I can tell my own experience: I have myself a pair of 155's and the 101C for the central channel and until recently I've always looked for a pair of tiny surrounds with some degree of matching sound with the fronts. I only had a pair of Cambridge Lusso 10 for the rears (no matching sound whatsoever). Last week, I installed a bookshelf behind the couch and I'm now testing proper bookshelf speakers (some old Pioneers, and Tannoy Mercury v1). Even though they are not the matching speakers of the fronts in terms of timbre or sound signature, the difference with the little full-range speakers is huge!. I know it sounds quite obvious for most of you, but the more alike the speakers (even with the rears), the better. Even if they are used only for movies, you would feel a more inmersive experience because the sound will be more detailed and rich.
Now I'm looking to buy the 210's (easier to get as the 100's series is outdated).