The fuse is what protects the wire. If you use too small of a wire, but bigger fuse, you can melt the insulation. It's always better to us a little smaller fuse, but keep the bigger wire and just keep spare fuses around. 30 Amp fuses are the right size for 10 guage wire. I would bump it up to at least #8 wire if i were you and put in a 40 AMP fuse, you'll get better efficiency out of your batteries and TM, plus you won't blow fuses as often.
If motor guide says 50 amp breaker, put in the # 6 wire and 50 amp breaker. They made the TM and know what they are doing, G3 made the boat and put in smaller wire to skimp on the cost