When I wear the traditional kilt... I wear the traditional belt with the rectangular buckle.

When I wear the casuals... I wear a 1.5" belt with any number of buckles. I actually use the buckle as the "defining" accessory. If I'm in a rowdy Pub, then I'll wear the one with the bottle opener in it. If it's a nice place, the Celtic Cross works well. I have a LOAD of buckles that run the entire gammut of "buckledom". Sicne the kilts, sporrans, shirts and boots are pretty-well set, the buckle is the ONE THING that I use to chenge it up.

As best as I know, there are no rules about buckles... unless you're dealing with a kilt belt and formal wear.