Looking into Kribensis it says the tank should be 3ft if I'm keeping them in a comunity

my tank is only 2.5 ft (at my calculation)
Also reading into it it says they need dark substrate so looks like a no go
Probably because they get quite aggro when breeding, just like any cichlid type I suppose. Dark substrate isn't necessary, it just shows off their colours better. I had my pair on pale sand and bred them a few times before moving on.
You could add either more flame tetras to up the shoal, or add a second shoal of something. If you want to stick with red then you could go for something like cherry barbs or hengeli or espei rasbora. Alternatively, if you want something that just looks nice, you could have a pair of killifish in there, or a shoal of cardinals/neons (regular, green or black) depending on tank temp, rummy nose tetras or lemon tetras.
All depends on what you want in there, whether it's a centrepiece type fish or another shoal to watch.