We used a 6'7" West Marine (Bombard) with slat floor for several years on the C Dory 22. For one person, it was fine. For two people it was satisfactory for short distances. For two and a dog--it became marginal. My son (6' 4", 220 lbs and myself 6' 2" 190 lbs) was too much--but we made it back and forth to the dock in calm water. The soft floors are difficult to get in and out of, the slat floors are easier--solid plywood, air or aluminum floors are much easier.
We went to an air floor 9 foot boat, because with the Tom Cat 255, we will be taking the dinghy much further than we would have with the CD 22. The air floor will plane--and we may go with an engine large enough to plane the inflatable. Also we have a davit to lift the dinghy aboard and even put it on top of the cabin (we have not yet rigged the mount for that yet--but do have it for the side and stern).
If you are going to row, I think that the rounded boats are fine. For an engine I prefer the transom type. (I had several Avon Red Crests, Red Seals etc in the 60's and the motoring was not all that satisfactory, but it will work for short distances.)
As long as you are only going short distances in protected water--anything (sturdy) will do--but remember if you get caught in heavy wind, that you can not row against it--or even a 2hp will not be enough.
Bob Austin