800gph is good, but when you can, maybe upgrade one or both filters and it will be even better. 10x filtration, in your case 900gph for a 90 gallon, is what is usually recommended for messy fish like goldfish and oscars. With multiple oscars, two plecos, and other cichlids, the more filtration the better. It isn't really needed now, but once they get a bit bigger you will want all the help you can get until you get that upgrade. And if it is ammonia burns, it is possible that the damage was done before you got them and you are just noticing it now, or if it was in your tank, perhaps they are just slightly more sensitive than the other fish. Did you cycle fish in, or fishless?