Fluvals are gravity fed. THe height of the water would have to be the same in order for gravity to have the same pull. Otherwise the second filter will lack as it does not recieve water as quickly as the first. Pumps can be perfect sync, but still won't work evenly.
As to the discus, I'd say 6 in a 55 is fine. I've seen 10 in a 75, that are more than beautiful, from discus breeders. The main thing is that water quality stays up, if you are willing to do huge changes daily, then no probs. If not, then more water. As with most fish though, the more space they have the happier they will be.