It would be easiest if you called and asked what breeds they had or if they don't know, ask what hatchery they ordered from and you can check online to see what breeds they carry... then you can see pics of what the different breeds of ducklings look like and pick out the ones you like best.
I had to call the hatchery when my local feed store got in goslings because the girl i first spoke wiht wasn't sure.
All of the TSC's around me have pekins. I know someone that got 10 ducks from them, they seamed really healthy, only one duckling was lost and it was due to a fox. Make sure before you go to get them that you have a proper brooder set up. Put the water in the brooder and turn on the light before you leave so that way the water will be at the eperfect emperature for them. Post some pictures of them if you can.
My neighbour last year went to TSC and bought 10 ducklings. She ended up with 7 mallard drakes and 2 female drakes and an odd duckling who she thought was cute. It turned out to be a black and white Runner mix with a crest..who also turned out to be a drake. She still has them all on her pond and they come up to the house once a day to be fed then back they go to the pond
Thats been my experience a few times. I have went and got ducklings from TSC and Atwoods both said to be from ideal. I had some birds that were clearly crossed but then again i had a few birds that matched APA guidelines and won Grand Champion. Just my 2 cents.