I have some 30's with 1000' and there looks like some space left on the spools but not a lot.
Don't think I would want to use a smaller reel though. Bigger reel - bigger drag.
Wire hits are violent and when the drag screams off a smaller one may not take the abuse for long.
Always seem to be cutting off the end of the wire because it gets coils like pigtails after a while. I would think a smaller diameter spool would be even worse for that and there would not be a lot of wire to spare for cutting.
Probably also depends on how and what you are fishing for. If you typically run just a small dipsy back 150. Ft for bows or maybe a 124 deeper diver back 400 ft for the big guys. Add 400 or better for the run of a big king and you can see where 1000 ft isn't a lot.
You will spend 1/2 the cost of a 30 for the wire to fill the 20 and first time you get spooled and have to replace the wire you have paid for a 30.
I would save the trouble and start off with a larger reel just my thoughts. Mike