I'll answer the last question first because I know the answer. Yes you can combined 635 and 650. Actually I already done this. The only problem is getting high quality dichro which will cost you. It's about 200$.
First question is as old as laser diodes themselves. Once you cross 200mw mark beam start to get worst and worst. If the diode makes an oval shape beam then you can compress it which 2 prisms. If it makes square beam then all you can do is to use a telescope. Strait it will act like a collimator (means exit beam will be very very fat and will not fit on galvos but will give you a very nice beam @ focus). Or you can do reverse telescope which will give you a nice beam at the exit but fat beam @ focus. Getting as big galvos as possible (6mm or 8mm) and playing with telescope can get you OK beam. But it'll not be as good as for example 671.
I hired an Italian guy to do my wires. Now they look like spaghetti!