How big, in feet by feet, is 'good sized'?
When will the building be ready for birds?
Does/will it have power?
Mrs. K has some good points...especially putting the chicks in there first.....or all the birds at the same time to avoid the territoriality issues.
All birds do this
when their territory is 'invaded' by new birds.
Depending on your coop size and configuration, I'd strongly suggest you set it up with a segregation area by accommodating a temporary wall installation.
This is how I integrate my new chicks with the flock.
https://www.backyardchickens.com/articles/integrating-new-birds-at-4-weeks-old.72603/
This is a safer heat source for chick, in the house or the coop:
https://www.backyardchickens.com/articles/pseudo-brooder-heater-plate.67729/
Here's some integration tips, sorry if you've seen them before.
Integration Basics:
It's all about territory and resources(space/food/water).
Existing birds will almost always attack new ones to defend their resources.
Understanding chicken behaviors is essential to integrating new birds into your flock.
Confine new birds within sight but physically segregated from older/existing birds for several weeks, so they can see and get used to each other but not physically interact.
In adjacent runs, spread scratch grains along the dividing mesh, best if mesh is just big enough for birds to stick their head thru, so they get used to eating together.
The more space, the better. Birds will peck to establish dominance, the pecked bird needs space to get away. As long as there's no copious blood drawn and/or new bird is not trapped/pinned down and beaten unmercilessly, let them work it out. Every time you interfere or remove new birds, they'll have to start the pecking order thing all over again.
Multiple feed/water stations. Dominance issues are most often carried out over sustenance, more stations lessens the frequency of that issue.
Places for the new birds to hide 'out of line of sight'(but not a dead end trap) and/or up and away from any bully birds. Roosts, pallets or boards leaned up against walls or up on concrete blocks, old chairs tables, branches, logs, stumps out in the run can
really help. Lots of diversion and places to 'hide' instead of bare wide open run.
This used to be a better search, new format has reduced it's efficacy, but still:
Read up on integration..... BYC advanced search>titles only>
integration
This is good place to start reading, BUT some info is outdated IMO:
http://www.backyardchickens.com/a/adding-to-your-flock
Hello, everybody! So, I'm getting new chicks coming in late Feb. I know my signature says "March", but I moved the date up. If you read my signature, it explains all of the breeds I will be getting. But now, I will do the amounts. 3 Welsummer, 2 Buff Orpington, 4 Barred Rock, 2 White Leghorn, 2 Golden Laced Wyandotte, 3 Americana, and 2 Sapphire Gem. I will be adding these breeds to an existing flock of: 8 ISA Brown hens and 1 HUGE Buff Orpington roo. We are getting a new, good sized building, so I will have to move the ISA Browns there first. The babies will leave their brooder at six weeks. What is the best way to integrate them to my girls and roo? I've heard that ISA's act like this to newcomers 
. Also, I've heard that the hatchery I'm getting my chicks from, (Hoover's) sends "surprise" roos. How should I integrate them to my VERY nice BO roo (if I get them)? Thanks in advance!
