"is it ok to use a bmx cankset with a track frame?"
"what do i need to know about the spacing?"
you'll need to know your bb shell spacing to get the right bb. you'll also need to know the bcd of the cranks if you get the chainring separately. if you get the phil bb, you won't really have to worry about spindle length, seeing as how it's adjustable.
"i was thinking of getting a phil bb. should i just get a pista crankset?"
that's up to what you can afford. a track crankset would be best and easiest, but you can easily get a bmx or road crankset to work as well.