Good questions!

Personally, I divide my kilts into casual and traditional, regardless of whether they are plain or tartan.

I keep the casuals, mostly 4 yard kilts, for casual wear and the tradtionals, or 8 yarders for formal. This is only because generally speaking, the 8 yarders are better quality.

Other factors determining my choice would be the weather (wool or PV), colour matching, ie. if I have a jacket that would go better with a certain tartan kilt, then I'll match then up.

Overall, most kilts are flexible enough to be worn in any situation, that's the wonder of kilts. What matters more is how you accessorise the kilt, wear a T shirt for casual or collar and tie for formal.