My Rams and Betta were fine together, before I got the Betta it's own tank. There was never even a sniff of fighting/flaring and the Rams spend most of their time along the bottom, which was the opposite of the Betta.
Saying that though, each fish has their own personality and some are more aggressive than others. Providing you have a back up plan if things did get nasty, then I'd give it a go if it's what you wanted. Kribs ont he other hand... my experience of them is that they were nasty little sods, breeding or not.