Howdy
claracluck1180 and Welcome to BYC!
While the size of the yard is important the size of the coop has to be factored in also; while your yard will accommodate 8, will your coop? Your climate can also be a factor in that, where I am located, the warm climate and covered run means that the chickens are only in the coop to lay an egg or sleep but if your climate dictates that they will be spending days at a time in the coop they do not want to be cramped as this will cause bickering, boredom, feather pulling etc.
If you do bring in new pullets, it is advisable to quarantine them for probably at least two weeks [some prefer longer] so that if they do have something nasty, your existing flock is not at risk.
Personally, I do not try and integrate anything under 10 to 12 weeks old.
I use the ‘look but not touch’ method on integration [after quarantine], meaning that the newby(s) are separated from the existing flock with wire for a couple of weeks so that they can see and communicate but not feel threatened or bullied. Not sure if you can do that with your existing set up?
After a couple of weeks, while they are still separated by wire when in the run, free range is supervised and all together; space is a wonderful aid in integration.
Another trick I use is to keep the existing flock locked in the coop/run and let the newby(s) free range .. this gives the newby(s) time to explore the garden without the threat of being picked on and also to find all the hiding spots and feel comfortable in the surrounds. Granted, the existing flock are not impressed but they get over it
I also switch it around in that I let the existing flock out but keep the newby(s) locked up, giving them a good chance to explore and become comfortable in all of the coop/run without being picked on.
Sitting with them and sharing out treats also helps take their mind of the intruders.
Good luck with your new gals, I hope it works out for you.