It may or may not be good to scan as a colour positive, that you will have to determine yourself. The gain may not be worth the extra filesize (in most cases it is not, in my opinion, but I have only worked with agfa scala, not with your Foma...)
Go ahead and scan to colour, 8 or 16 bit per channel or whatever. Then open up in photoshop, and go to Image/adjustments/channel mixer....
You can work on each channel separately or together. Working on the channels separately can be useful if you note that noise/grain is more evident in one channel. This is often done, for example, with b&w negs developed in staining developers.