Have you tried the combo before, Arowana?
I've tried the combination with no issues. The only trouble is clown loaches are rather inquisitive fish and may trail around the discus much to their fright. This usually happens in cramped quarters so if your tank has plenty of space while permitting plenty of hiding places, it should be fine. I'd assign an area for currents as well for the loaches to swim along. They originated from fast flowing rivers and shouldn't be deprived of the acitvity they truly enjoy.
All strains of discus are still the same species so yes, they can breed with each other. For a 90g, I'd consider 6. If you're starting with juveniles, make the tank barebottom so maintenance is easier. They do tend to require plenty of feedings which subsequently requires plenty of water changes to keep water conditions on top.
I agree with Knowdafish. Forget the feifengs. They are already incompatible with stingrays. Discus are no different as these secrete slime coat which could become the protein source of most suckers such as plecos if deprived of proper food diet.