A lot of 28' toons are I/O powered. That would darn sure make a difference in what the proper trailer looked like. Same story is transporting with a motor installed, or no?
I've pulled pontoons all over creation and the biggest thing I can share is that they pull like a parachute. I would plan on making that trip at no more than 60-65mph and that's using a pretty heavy duty tow vehicle. Allow plenty of time to cover this 400 miles.
When you say 25-26' trailer, are you talking about a trailer with bunks that long (the things that actually support the boat structure), or is that the total length of the trailer? A trailer with 26' bunks should be OK, but the big question remaining would be tongue weight, whether or not the boat could be located on those bunks to give you enough tongue weight to get proper handling. Without actually loading the boat, I don't know that you're going to be able to predict proper tongue weight one way or another. Might be fine, might not...
To make it right if it's not right, the axles are moved forward or backward on the frame. Back increases tongue weight, forward decreases. Point being here, this trailer you rent should be easy to adjust using a minimum amount of tools.