I have a 7.x Magnepan system. I use MG-IIIa's (old, old Maggies) for L/R. they are only about 18" - 24" from the wall behind but that wall (and all others) are heavily treated so no comb filter effects. A stereo pair of Rythmik F12's sits beside the mains, after an active analog crossover. Center is a CC3 and the surrounds and rears are MC1's. The MC1's are only inches from the wall and seem to do fine; hair more bass, and cancellation effects I do not seem to notice for surround and back material. Speakers are driven by an XPA-2 and XPA-5; frankly, my Pioneer SC-27 drives them fine but I already had the amplifiers. The room is not large (about 13.25' wide, 17.5' long, 8.5' high after hanging the ceiling from Kinetics IsoMax clips for isolation).
I have mixed other speakers for surrounds and it was OK. I needed them for another system and went with Magnepan surrounds/backs sinec I was able to afford them and had a plan for their installation (using DIY stands). Maggies all around sound great (natch!) but I would not let that deter me from saving money and space on the surround speakers. I do prefer the sound of a Maggie center, however; it makes the transition across the front stage more seamless.
Most planer speaker like Maggies and ESLs, or really just "most speakers", exhibit quite high distortion when driven by loud LF signals. I much prefer to use a good subwoofer and integrate it with the mains, rolling them off to preserve their performance and amplifier headroom where it is useful. I do not own 20's but have heard them a number of times and would still use a sub with them. YMMV.
The 3.x series and above adds a true ribbon tweeter. When I got mine the difference was significant and I much preferred the big ribbon tweeter. I have not heard the newer models enough (hardly at all) to see if the step-up from quasi- to true-ribbon is as large a difference today. The other difference is the larger panels provide a larger sound stage and bit deeper bass with a little more LF output.
Magnepans are nominally 4-ohm speakers with the top end on the ribbon models falling to perhaps 3 ohms. However, the impedance does not have the large excursions in impedance magnitude or phase that many conventional speakers have, nor the extremely low HF impedance exhibited by ESLs, so is not as difficult to drive as you might think. They also behave as line sources over much of their frequency range (upper bass on up). That means SPL fall-off with distance is less than with conventional speakers, increasing their relative efficiency/sensitivity at the listening position, and less interaction with walls, floor, and ceiling (first reflections not a big deal). IME room treatment for planers should start with the wall behind them, then the opposite wall (especially if the rear wall is near the listening position). Sides and ceilings last if needed at all.
Often times people come to listen and ask me to turn them up. Sometimes they just want it louder, sometimes they have bought into the "crank them to hear them sound their best" myth. The usual result is shock and awe when they turn to tell me how great they sound only to find out they can't hear themselves speak.
Clean sound is a wonderful thing.
All IME, IMO, FWIWFM, YMMV, my 0.000001 cents (microcent), etc. - Don