If your weather is warming up now, this is a great time to get them out into the coop. I usually get them out sometime between 5 and 6 weeks old. If your nights are still sort of cold, you can take your heat lamp out with them for a week and use it only at nights just in case they do need some extra warmth yet. Don't keep it really close, but close enough it takes the chill off. Make sure the lamp is permanently attached to something so it can not fall and start a coop fire.
Have you stopped by our learning center yet? Lots of good reads on all the aspects of keeping your new flock..
Hello and Welcome To BYC! You've gotten some good advice above, X3 generally when they are fully feathered at around 6 weeks and the weather is above freezing. X2 on the BYC Learning Center, lots of good articles on all aspects of chicken keeping.
It is generally best to wait until birds are about the same size before putting them together. If you put six week old chicks with adult hens the adult may attack and kill the chicks. A long period of seeing but no touching through wire seem to work best, usually if you can divide the coop/run in to parts or put a wire cage etc in for the younger ones. The chickens will get to know each other and sort of work out a pecking order. There is a nice article in the Learning Center on integrating flocks you might like to check out, the part about actually combining them is after the quarantine section https://www.backyardchickens.com/a/adding-to-your-flock