You can thin seedlings at just about any time depending on how careful you are. Some prefer to wait until they are at least 1" tall just because they are easier to separate.

If you have fat thumbs and fingers like me I prefer using snips to thin rather than pulling them as it doesn't disturb the roots of the others that I'm leaving.

As to how many to leave per square, that's a sq. foot gardening issue so they would know best. 1 per sq. foot sounds about right to me but in really fertile soil some would probably try to squeeze in 4.

Good luck with you broccoli. I hope the weather cooperates with you planting it this late.

Dave