I can report now that two females of different size get along, even in a 10 gallon QT if it has enough rocks.
The little one got it's tail fin a bit shredded on the second day but it healed within two days and now the hacking order has been established. The bigger one still come up to the small one once in while to demand submission but when the little one shows submission the big turns away and leaves it alone.
So as long as you put a larger female to a smaller it should work out fine.
Of course it will also work if you pair a female with a larger male.
But since it may for most be a bit difficult to tell males and females apart, pairing two females (specimen under ~4") is easier.
Also, if the tank size permits it, you don't need to stop with two but put a few more (3 to 4) together. Though in that case I would go with several really small ones (below 2", ideally tank raised) and one significantly bigger one.