Congratulations on the "new-to-you" trailer, and welcome to The Forum! I, also, have two 25'ers. But since you already have the 50, go ahead and cut it up. The fittings aren't that expensive - just make sure they're food grade. The lengths you suggested sound good to me. As an alternative, you could get one 25'er, then cut your 50 down to two 25'ers, wrap some black electrical tape around each end of one (as a reminder), and use it for your black tank flush hose. At least it won't go to waste.
(Oh! I guess if you use it for you black tank flush, it will go to "waste".)