Firemouths are awesome, and a pair would fit in a 65, and would probably breed readily, make sure your dithers are fast fish, the firemouths will kill anything that they feel is a threat to their young.
My 67 gallon will have Angels rams and tetra's along with cories, and a lot of plants, and maybe a gourami or two ( still in the planning stages) to me that is the ideal size tank for plants and angels unless you want a large school of angelfish.
You could also go with convicts or with the africans as mentioned. I would keep my stocking ideas to fish that don't grow past 6".
As far as the Oscars and the 180, you are pretty well stocked, 15" inches isn't a norm, but they can get that big, I'd get a pleco and a couple of pictus cats or a glass cat and call it full. I'm not a big fan of most Dithers that will work with Oscars, If you only had one or two Oscars I'd suggest pearl or blue gourami's or a severum. with three you'll be crowded enough.
Dave