I take it all your fish are fairly young, and because of it, the tank appears to be sparsely populated and large. In my opinion, a 90 is not large, it is about the size of a water filled truck tire rut in in the road.
When the knife gets up to 20" if healthy, would in my opinion max out he tank by itself.
As would the Uaru, barbs and fests in another adult stocking scenario.
But at 20" being a nocturnal feeder the knife mayl solve some of your stocking problems by it self, meaning, while the smaller cichlids like the Kribs are resting, it will eat them. but before that, will try to probably eat the corys.
Geo's might out last some of the smaller species, but they do require high quality water, so as adults (the ones that aren't eaten) will need a more frequent and heavier water change schedule to remain healthy, in a then crowded tank..
If you do what you suggest, it will be an interesting experiment to see what happens, and I hope you keep us updated as to whether it works or not.