You're all definitely correct. Goldfish will indeed eat anything and everything they can fit in their mouths. If you're worried about the smaller fish and you can't afford a larger tank - or the 10 gallon doesn't work out - you can put the otos and small fish into a floating breeder net temporarily.
I disagree with watauga in that I think it's important for goldfish to have something to sift and spit. If your gravel is getting stuck in his mouth, switch to a larger or smaller substrate, like sand, until he gets bigger. If he swallows the sand, it will move through his digestive tract without getting stuck. You can buy large bags of masonry sand very cheaply at most hardware stores. Rinse well, and you have new substrate.
I do agree that in goldfish tanks, decor should be kept to a minimum. For the singletail, torpedo-shaped varieties, they need as much swimming room as possible, and for the fancier varities, they require a tank without any sharp or rough decor due to their long fins and slow movement. A few plastic plants, a piece of driftwood, some substrate. That's really the most I would recommend.
It's pretty common for goldfish to get things stuck in their mouths, but if it happens repeatedly you could try moving or removing all the mouth-sized objects. Good luck.
