I assume when you say CSW it's Cambridge Soundworks? There's a name from the past. I just checked to see if they were even still in business, and surprisingly they are. They're pretty much selling the same things from 10 years ago though, which isn't encouraging.
I found the MC305 on their website but didn't see an S300, so I can't tell if they would match up properly. But looking at what they sell -- which is mostly stale old products -- I think you can get better from other manufacturers. Probably just about any other manufacturer. Be that as it may...
Mix-n-match is not generally a good idea, at least with the fronts. They need to be tightly coupled, and will usually highlight a mismatch by producing a less than coherent soundfield. Whatever you decide to buy it would be best to match the front 3 speakers.
