how to select healthy chicks from the local feed stores?

Hi I had planned for a certain number of hens when I made my chick orders, and ended up with more roosters then planned, I just got in 5 chicks and it was too late to add to my order when I realized my rooter count, I wanted to get 2 more chicks to go with my 5, 3 day old chicks. So my question begins: There is a feed store near me that has chicks in and is the only one selling chick right now, they have been sexed and vaccinated by the hatchery that they came from and should all be pullets, What worries me is the presents of pasty butt, some have it bad and other seem totally clear, I believe that because of the large number of chicks they are not addressing it as they should but am not sure I noticed some brooders look over all better then others. Should I pick out ones that have clear vents or skip them all together?
I had one in my last batch that was very stress from the travel and got pasty butt, I cleaned him immediately and didn't have any recurrences, the other brood mates were fine. I don't know too much about raising chicks and don't want to risk making my current babies sick. Also is it safe to add in week old chicks to the few day old chicks?
thanks

I'm not expert, but think it would be better to raise them with only a week apart in age than to try to introduce more when they are older.
I would pick ones that are more active in their brooder and without pasty butts. I would hate to have to double stress them with a ride home and immediately have to clean butts.

I think that it would be possible to get the feed store chicks without seriously endangering your current birds. When picking them out, be sure to choose ones without pasty butt. Choose active chicks that you see eating and drinking well. Don't pick chicks that are sleeping, hiding in a corner, or have pasty butt. If any of the chicks at the feedstore look like they have a contagious respiratory disease, skip the feed store chicks altogether.

Ideally, you'd want to at least temporarily isolate the chicks when you get them home, just to make sure that they don't have a disease. Or, you can take your chances and introduce them immediately into the current group. Be sure to watch them closely and monitor them to make sure they are all eating and appear healthy.

Mixing one week old chicks with chicks a couple of days old should be fine, as long as there are no particularly weak chicks. I've successfully kept chicks of different ages together before, and others have too. Just be careful.

okay thank you for the advice!

I buy most of my chicks from the feed store. I try to get them as old as possible, to make sure they're the stronger, healthier birds of the batch. Look for alert, active, bright eyes, no pasty butt, things like that. Do NOT, under any circumstances, "rescue" a chick that's doing poorly! There's usually a reason it's not doing well, just let that one be and go for the healthy birds.