Start dialing down the heat lamp use...watch their behavior:
They need to be pretty warm(~85F on the brooder floor right under the lamp) for the first day or two, especially if they have been shipped, until they get to eating, drinking and moving around well. But after that it's best to keep them as cool as possible for optimal feather growth and quicker integration to outside temps. A lot of chick illnesses are attributed to too warm of a brooder.
The best indicator of heat levels is to watch their behavior:
If they are huddled/piled up right under the lamp and cheeping very loudly, they are too cold.
If they are spread out on the absolute edges of the brooder as far from the lamp as possible, panting and/or cheeping very loudly, they are too hot.
If they sleep around the edge of the lamp calmly just next to each other and spend time running all around the brooder they are juuuust right!
You most likely
will have integration issues:
Here's some notes I've taken on integration that I found to be very helpful.
See if any of them, or the links provided, might offer some tips that will assist you in your situation:
Integration of new chickens to 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.
Integrating new birds of equal size works best.
For smaller chicks I used a large wire dog crate right in the coop for the smallers. I removed the crate door and put up a piece of wire fencing over the opening and bent up one corner just enough for the smallers to fit thru but the biggers could not. Feed and water inside the crate for the smallers. Make sure the smallers know how to get in and out of the crate opening before exposing them to the olders. this worked out great for me, by the time the crate was too small for the them to roost in there(about 3 weeks), they had pretty much integrated themselves to the olders.
If you have too many smallers to fit in a crate you can partition off part of the coop with a wire wall and make the same openings for smallers escape.
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 blood drawn and/or new bird is not trapped/pinned down, 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 and/or up and away from any bully birds.
Read up on integration..... BYC advanced search>titles only>
integration
This is good place to start reading:
https://www.backyardchickens.com/a/adding-to-your-flock
I can't believe how cold it is here already!! Low 20's tonight!!
