That's a good retailer! Most aren't smart enough to know about, much less advise on, battery safety!
Yes, you should buy batteries JUST for that mod, label them in a way you will know they go together. You can use a label maker, a permanent marker, fingernail polish, whatever you have handy.
The problem is no one knows how those batteries have been used and abused, how many charge cycles, how many deep cycles etc, any or all of them have been through. One weak battery in the set makes them all that weak!
Sorry about the added expense, but what are your hands and face worth?