Chicks may not be available in the stores, but they should still be available from Craigslist and from the hatcheries.
I like Ideal Poultry, which has a dollar-amount minimum rather than a number of chicks minimum, reasonable small-order fees, and which is in Texas so their breeding flocks are...
It has been blistering hot here for sure. :(
I'd suggest not adding until you get your coop fully in operation, but late summer/early fall isn't a bad time to be adding chicks in our climate. From late June to early September is not a really good time because of the heat.
Yes, once they are all laying they will be more willing to associate with each other.
That's a definite maybe.
*Sometimes* roosters will get along -- I have a senior and a junior right now who are getting on well. But there are no guarantees and it could change in an instant.
It's less...