1500 sq ft seems a bit small Jonathan.
I've been to a few dance holidays at a hotel with a 1500 sq ft ballroom and everyone complains that it is too crowded with more than 100 dancers.
One monthly dance here in Ipswich, UK, regularly gets 250 to 300 attending in a sports hall containing four badminton courts. Sorry, I don't know how big that makes it but it's bigger than 1500 sq ft, and it's pretty crowded with 300.
As you say, much depends on the standard of dancer and how keen they are to all dance at once. I'd go along with your estimate of only 2/3 at any time for general dancing.
Six top competition couples could fill 1500 sq ft but six typical social couples will get bunched up in one corner. 
Howard