No flatbottom or mod-v is going to like open water when the wind is blowing much. My 18' mod-v does OK up to about a whitecap, after that it is just a brutal ride.
My fishing buddy has a 1548 G3 w/20hp Yammy on it, and it is a fishing machine. So yes, they can work just fine with the smaller motors, if a 20 is considered smaller.
As for the trailer, you can usually get a trailer tagged as homemade even if it's not. In doing that, you don't have to worry about tickets. People do that because it's often easier than transferring paperwork on a trailer or the paperwork got lost along the way and they don't want to mess with all the red tape. If your county is a stickler on the rules, then it might make getting a trailer tag more difficult.
If you're not living on the edge, you're taking up too much space!