Jim,
It sounds like you have acquired a "bass boat" with a raised casting platform up front and if that is the case, rod length would not be your main problem to manage. Your main problem would be fly line management. Trust me on this because "been there & done that". Since you have a foot controlled trolling motor, you will have a constant battle with the fly line constantly getting tangled around the foot control and any other obstacles on the front deck. If there happens to be an anchor rope cleat on top of the sides of the casting platform, you will need to remember that when you lay the rod down across the side it will slide down and stop just under the end of the cleat and if you are not careful, when you pick up the rod you will break it. Don't ask me how I know that......
A 9' rod will work out great and I do not feel you will need anything longer. The fly rods being offered now for bass fishing from a boat are offered in a length no longer than 7' 11" so that they will fit in the rod box plus meet tournament regulations of length. I think your distance casting will be affected by a 7' rod and feel that an 8' or longer would be best.
Just my thoughts and nothing more....
Warren
Fly fishing and fly tying are two things that I do, and when I am doing them, they are the only 2 things I think about. They clear my mind.