Hi welcome to BYC! I just put in an order for some chickies this morning, and would be more than happy to sell you some of the extras. I got a super neat assortment.
Mixing different breeds is fine, as long as you don't overcrowd them. You are going to love having chickens!
Buy a copy of Uncle Henry's. There are a number of older chicks and some laying hens for sale in there. Part of the fun is starting from day old chicks so I suggest you really consider doing that. My first 5 chicks last spring were 2RIs, 1 Austrolorp, 1Buff Orp, and 1 Easter Egger plus a RI Cockerel. I bought 4 more RI chicks the last week of August. Merging the two groups happened wasn't easy but they became 1 flock in the winter.
You are in for a real treat.