Personally I wouldn't recommend getting another, it may end in both gourami fighting and being injured.
Maybe rehome the kissing gourami or pleco.
Even if it looks like plenty of room to you, that doesn't mean that it is to them. You should always stock for the tank size you have, not the one you plan to upgrade to in the future.
Why not move them to a pond now?
Unfortunately, if you're going to get a big fish, you need to be able to afford a big tank for them. This is really too small even for a grow out tank for them
Staying in the 20 gal until they reach 7" is way too long.
He was really young then, and it's good you're fattening him up. I just think you'll have health and stunting problems if you wait until they're 7" before moving them to a pond.