Kribensis don't need a high pH. They're an African RIVERINE species, not from the rift lakes.
Anywhere from a pH of 6-7.5, as long as its stable they'll be absolutely fine.
Chances are a pair will spawn if they're compatible, and if they do it'll be better to not have any other bottom dwellers.
Chances are in a 30 gallon tank other bottom dwellers would be fine but IMO not really worth the hassle if the pair gives them a hard time.
As far as tank mates goes, aim for larger tetras, round body tetras (lemons, red eyes etc), platties and swordtails, and barbs (tigers, black Ruby's (if you want colour this fish is for you), Odessa barbs etc.