Is it a normal mono sound you get if you use a M/S stereo mic by only plugging in the left or center? Does it sound like a normal mono shotgun mic?
SOME M/S microphones provide discrete access to the Mid and Side outputs, but most provide only the matrixed Left and Right derived signal outputs.
Your question cannot be definitively answered as a generic question. It depends on exactly WHICH M/S mic you are talking about and whether it provides Mid and Side outputs?
Specifically, the Sennheiser MKH 418S has separate line-gradient ("shotgun") MID microphone and a separate figure-8 "side" microphone. It seems quite reasonable to expect that the "MID" shotgun section can be used independently without powering the "side" microphone section.
Yep you certainly can just use the centre on a 418! Its similar to a 416 if you so this.
Yes. Absolutely. Mono-Compatibility is exactly what Mid/Side recording is designed to accomplish, in fact quite a lot of Mid/Side recording rigs actually consist of a standard mono shotgun condensor mic with a figure-8 microphone gaffered to the top. Works extremely well. Of course you could go for a 418s - type solution where you have a shotgun mono Mid mic integrated with a figure-8 side mic in the same tube, or you can go for the redneck repairs style of solution. Both work equally well.