I prefer a whitewater style paddle for river fishing. I tend to drift, then make a couple of quick strokes then get back to the business of fishing. I am currently using a werner desperado that is 207 cm long in a 28" wide Manta Ray 14. I previously used a 200 cm whitewater paddle but it was a tad short.
If your cruising on the lake or slowly strolling down river (rather than rapidly moving to avoid something), then Troutman recommendation of 220 cm (plus or minus 5 cm) is a good starting point.
A shorter paddle is a lighter paddle, and you will notice a poor paddle long before you notice a poor boat design, IMO. The whitewater style blades are wider so that you get more bite, which helps with turning (along with the more vertical stroke). The touring blades tend to be longer and you use a wide sweeping stroke (with a slower cadence).
Get to a good paddleshop and test out some paddles while sitting in a kayak. Even better get to a demo day and test on the water. Lots of variables between grip width, torso height, seat height and posture, and just personal preference.
As far as seats go, most work with all boats. You might have to change around the attachment points though.