All my goldies have grown so big.

Goldeen now looks like a koi but isn't. He/she picks on Mr. PoPo terribly (Black Moor). He/she needs someplace to hide until I move everyone to a bigger tank after I move at the end of next month. We had a little cave from the pet store but no one can fit inside it anymore (Do they make bigger ones?). I was thinking a ceramic pot or some kind of pitcher laid on it's side. Something very smooth that won't tear delicate fins or eyes.

They usually don't fight except at feeding time and then Goldeen chases Mr. PoPo and Callie. They will sometimes fight back when they've had enough or have an evening argument about who gets nearest the heater for the night.

Callie and Mr. PoPo never fight and  are around the same size/shape. Goldeen is more comet-like except with a veil tail and is soo much longer and more threatening to my other guys.

I'm halfway wondering should I bother to put them back together at all or keep 2 tanks and leave Goldeen in the smaller tank. They would be going from a 20 gal tank to a 40. Will it make a difference or will it make Goldeen depressed being without buddies even though he's a bully?

I agree with Goldiegirl My Horus was bullying everyone in the tank, even if he wasn't even the biggest! He just had the idea that the whole tank was his. No matter what tank he was in, so I kept him separated for a while, and he's been all nice ever since
That may not be the case with all fish, but do separate them! You don't want your poor Popo and Callie to get stressed and sick, right?