Started pullets are a great option! Definitely get a minimum of 2; introducing a single bird is a bad idea. Started pullets can range anywhere from 6-18 weeks depending on the seller. (If you happen to be in northern CA I know a great place).
Breed doesn't matter, though avoid more than 1 Rhode Island Red.
Introduction should be done over a period of 1-2 weeks. The best method is to let them sleep in a cage or crate inside the the coop during the night, and let them roam in a temporary pen (chicken wire and t-posts set up in one corner of your run are adequate) during the day. After 7-14 days you can wait until night fall and then place them on the roost beside your older hens. The younger the birds are the longer introduction should be; for example 16 weeks olds would do fine with 1 week separate introduction, 8 week olds should have at least two weeks.
And remember: pullets need a GROWER feed. Non-medicated and around 18% protein. If you buy pullets 8-14 weeks old, switch the entire flock to a grower and supplement your older hens with a handful of oyster shell in the morning. If they are 14-18 weeks, mix a bag of grower and a bag of layer 50/50 and feed this mix until the pullets are at least 16 weeks of age. At 16 weeks pullets can go onto a layer feed.
Any birds younger than 8 weeks should be penned separately until they reach this age or older.
And don't forget about quarantine! Any birds from an untrusted source, i.e. an auction, swap, or backyard breeder, need a 2-3 week quarantine before you let them anywhere near your birds! Quarantine must be done somewhere physically separate from your flock, like a laundry room, garage, or the opposite side of your house from your coop. Always wash your hands after handling birds in quarantine!
