My experience of boys and painting is my ds, who used to go out of his class in year 1 to do extra maths instead of the craft afternoon they had once a week. he thought he'd got a better deal. 
If he'd been invited to a painting party, I'm not convinced he would want to go at that age (he definitely wouldn't now!). If he had gone then I don't think the painting would have kept him engaged for very long.
I think if you'd asked me as a parent I'd have said for your own sanity don't invite him 
There is a gender split often at primary, although my dc always had friends of both girls and boys.
I used to see it with the swimming pool (yes, this was a state school) at the infants. We had a social time after school and I used to supervise. I did it for 9 years (summer term only as outside!). Almost every day (and it would be different children, so not that some children pushing their choice) you ended up with the boys in one group playing one big game and the girls split into smaller groups of 2-3. You could encourage them to join all together, but then they'd gradually split back again.
I think when a child is left out of a party once it is something they pick themselves up from, even if they're upset at the time.
Problem comes when it happens time and time again. That's when it hurts. And one of the issues is that if a child isn't at the parties, then they seem to get forgotten about because the other parents don't think of them.
One of mine got missed out in year R because she had a very similar name to another. And the parents kept saying "but I know there's only one Sophie in your form isn't there?" and not thinking to check if there's a Sophia.
I know this because it came out in year 1 when she had her party (early in the academic year).
OP, totally get what you say about space. Have you thought about doing 2 parties? Your dd will love having two parties, and you can have half at each. I'd not have them back to back, but maybe one in the morning ending with lunch and other in the afternoon. They can do the same painting at both.