If you did an all bichir tank you could do many more than two. You could even do several of them if you chose smaller tankmates than the cichlids you're proposing.
A 90 gallon doesn't change a whole lot, maybe waters down the bioload a bit - it's just a little taller but the length and width are the same. Going to a tank that is 5-6 feet long is what will give you more freedom in your fish choices.
If you do go with the heterospila, wet spot's are very small and have a rep for growing slowly as well. You'd have to be careful when intoducing polys (what we call bichirs here, short for their scientific name) so that they wouldn't be eaten. You'd have to size match appropriately or have a growout tank planned. Or get them first, and let them grow before adding the polys.