I wouldn't think twice about using mini's as surrounds. They're a great speaker and I almost feel like my Mini MK3s are under-utilized as back-surrounds in my system when matched up w/ 3SE MK3 surrounds/center, and 5SE MK3 mains. Luckily I was able to sell my Atom back surrounds for $165 and get my Mini MK3 surrounds for $105, so I got the best sound and made $$.
Either way, Mini's will definitely get the job done and 20's would be overkill unless you plan on doing alot of SACD or DVD-A listening.
Oh yeah... you may want to see if you can find some older LCR-450s or Compact Monitors. The LCR-450s were predecessors to the studio line and use the same drivers in an MTM setup. The Compact Monitors use the same tweeter as the studio line and were the predecessors to the studio-20s. There's a pair of Compact Monitors on eBay right now @ $92 w/ less than 2 days left...
I am using Studio ADP's (Surround L & R) and MiniMonitors Surround Centers and studio 100's up front, and it sounds great to me. The minimonitors are great speakers and can hold their own against much more expensive speakers.