Obviously at 1" you won't be able to sex them to get a "pair", though unless you end up with two males that take a strong dislike to each other (it happened to a friends pair and they had to be separated, on the other hand I had two females that would regularly "spawn" and guard the eggs together) you should be ok regardless of sex.
75g is about the bare minimum IMO for a pair, so I'm not sure how they'd get on with the payara, especially if they do breed for you. The other concern is if you're feeding meaty foods like shrimp to the payara they will more than likely go for that first instead of eating their greens, which isn't really that good for them. They are great fish tho, one of my favourite SA species.