Lewis,
I'd be looking at your image scale rather than f/ratio to decide if you've got the right set up for imaging galaxies. Of course, aperture will determine how long it takes to capture decent SNR
A FSQ-106 @ f/8 would be OK for the larger galaxy targets. With drizzle you'll get halfway decent resolution. It will need a fair amount of capture time to get a quality image.
If you want to go for more exotic targets then larger aperture is needed unless you have a lot of good sky time and patience.
Cheers,
Rick.