Want to add to my flock

[Natcho]

I have a flock of 8 mixed heavy breeds and two coops that they can use, their enclosure is very large and they often free range during the week. I've had them for nearly two years now and would love to add to the flock, I would especially like to get some chicks, but my mum seems to believe that 8 is the perfect number and any more would cause excessive bullying towards the chicken at the bottom of the pecking order, but I've noticed daisy is rarely pecked, apart from our very odd black tailed hen. Is my mum correct? Also I would love to get silver sebright bantams, would the size difference be an issue? And are they a good breed?
Thanks x