No, it isn't true. Less of a crest also is because of the quality of the chicken. In general, hatchery polish have smaller crests, but can vary. I've had hatchery polish with amazing crests and some with very sad tufts of feathering! Really, most Polish are not sexable until their crests mature to true feathers, and you can start to see their sickle feathers coming through. By that time the crest should be very messy and the sickles should seperate from the rest of the tail. Even combs are hard to say, because hatchery polish pullets often have abnormally large combs.
For SQ polish - I'm sure they vary too, but I don't know for sure. The size telling sex may or may not be true.