Hi aeparisi Welcome to Fish Lore
We're glad you're enjoying the forum and finding it helpful.
I think already know the answers to your questions, now to convince your partner you need another tank.
Your fish aren't compatable and you're over stocked.
The goldfish is a huge waste producer and the pleco is going to get big, fast.
The goldie is a cold water fish with different temperature requirements than the mollies and platys.
I'd suggest returning the pleco unless you're planning on getting a huge tank, it's recommended they have 55g.
You're goldie needs a larger tank also. He may be fine in a 10g by himself for a short time, but you'd soon need to upgrade him to at least a 20g.
Good luck.