They will start to pair and spawn around 2 inches, as far a compatability each pair is different. This pair is bonded very well and will rarely leave each others side, and defends their side of the tank fiercly.
A large tank and lots of breaks in the line of sight helps but inevitably there may be issues. What size tank are they in?
But the looks of yours is the female larger than the male? She may be testing the male to see if he is worthy of her, this can get rough and it's not uncommon for her to kill him if he fails her test or vice versa when he's ready to breed and she isn't.