You most certainly can use an M-20 speaker for your center channel. If you already have M-20s and are happy with that performance level then a complete system made up of M-20s and one or two subs should work quite well. Unless your room size is very large you should be fine.
I am not an advocate of the sideways center channel. I used to take this same advice lightly until I encountered a theater with three $5,000 center channels used for the LCR (Tall speakers would not fit the design). The manufacture said that the three centers would work just fine. I could never get the theater to perform very well no matter what I did. The poor sound was blamed on room acoustics; it could not possibly be the speakers I thought. After reading a thread posted here on Audiogon describing the "typical" dispersion pattern of most center channel speakers as being narrow in width and vertically high, I was convinced that the three center speakers were to blame. Imagine three vertical fan shaped sound fields bouncing off the floor and ceiling. Would you lay your tower speakers on their sides? Of course you wouldnt. If three center channels are really bad then why even use one?
The center channel ideally should be identical to the LR and have a wide dispersion. Unfortunately this usually is attained by stacking drivers vertically so that the acoustic energy of the adjacent drivers is cancelled out vertically, but maintained horizontally. This mimics the pattern that horn drivers throw. Most designs result in tower speaker like enclosure, which is not easily incorporated in most applications, nor is it very marketable from a sales point of view. Tall speakers don't fit under or on top of TV's very well. Of course the salesman is going to tell you that you "NEED" a center channel. It's an easy sale and seems logical to the consumer, after all it says "center channel" and fits on/under a TV.
Not all centers are bad. Some are well designed. It appears that the Revel center channel driver arrangement is not too bad. By using the tweet/mid combo they can get the crossover at a lower frequency limiting the bad dispersion effect caused by the flanking woofers. I can't say how good or bad they are. Judge for yourself.
What do you think the center channel looks like in a real theater? Hmmm... just like the left and right one;) Using all identical speakers is common advice, but you will rarely see it actually implemented in most HT magazines or showrooms.
I don't know if Revel will sell single speakers. If you can only get pairs look for a damaged pair for cheap or save the extra one for spare parts or future rear center. You could get M-20s for front L&R then buy F-30s and move them back as side channels later. The only drawback on the M-20 might be limited dynamics and bottom end. Addition of the sub should help here. On the plus side it is small and can be placed above or below the TV easily. Try inverting it if you are using on top of the TV to get the tweeter as close to the screen as possible. Personally, I prefer below the TV/Screen location if possible. Center content is mostly dialog and actors lip location favors the lower part of the screen. A low center channel is more easily convincing, plus it will integrate better with LR speaker location heights.