M. praecox grows to about 3"/70mm, as such, technically you could keep them in a small tank. That said, all the rainbow fish are very active swimmers and like open space which is not going to be possible in a tank that size. I would say it would be a bad idea to try them in there.
They are also a shoaling fish and should be kept in groups of 4 or ideally more. 4 of them plus a couple of Corys is going to fill the tank. You would be better off, IMHO, with Corys and some small tetras.