You would be just asking to kill your clown loaches.
Not trying to be harsh, but as already stated they like 80 degree or so temps and they're prone to ich. They're sensitive to water quality and in tank with massive waste producers like goldfish you'd probably have nitrate problems at the least.
They also get to a foot long, albeit slowly, so a 30g would be little more than a growout. Same goes for an oscar, they get between 12-16 inches long, and unlike clown loaches are not slow growers. Comets and commons also get a foot at least and can end up over 2 feet long, they're really not suited for less than a pond or very large tank as adults.
So basically you'd end up with several fish that are growing and would like to be fed a lot in a very small tank. Recipe for disaster, IMO.