its possible that because of space mine may have more issues. They currently have more than they need by far but it's a smaller space over all compared to the huge aviaries a lot of people seem to keep
My females are always aggressive about their space. You can take them all and house them separately in cat carriers or other smaller set ups for a few days to 'reset them' and add new ones with them when you put them back, but I usually just don't add new females to established groups.
I would not. Unless it's a male being introduced to a group of only females, I've never had any success adding one or two to an established group without serious fighting