By "those sucker fish," I assume you're referring to plecostomus. They typically grow to about 18-24". A crab I'm not totally sure about, but I've always heard that it's best to avoid them because they can hurt the fish.
Are you keeping the koi in a pond? When fully grown, it will need several thousand gallons minimum, and the pond should be at least 1 meter deep with an appropriate heater for winter.