Let see if I can cover everything here.
Yes dojos eat snails... but they will mostly go after small snails. I kept a large apple and ramshorn snail together with them fine. Just think that they have to be able to suck the snail out of its shell... with their itty bitty mouths. Thats the size snail they eat.
As to how well they do with different types of goldfish. They don't interact much. Sometimes they played... but I have kept ranchus, ryukins, orandas, fantails, telescopes and moors with them just fine.
Yes they will go to the surface for the occasional gulp, I had one who actually ate from my hand when handfeeding the goldies. They love to dig, so expect that. Plus they love to play in high current areas. Plus they can get up to 6 inches long if you treat em good

As to finding the loach on the rug. Yes, if they jump out they can survive for a while by excreating a protective coat. But if they dry out they will get infected on the exposed area and die. I have 3 jump outs during a move, 2 dojos died and one survived 12 hrs out of the tank.
They get along with bristle nose plecs, not sure on the otos, I think they will leave them alone anyway.
OH and you want to get a few. They like to be together with at least one buddy.
Goodluck!