Some really good LFS will house any new fish they receive for a time (1 week; sort of like a quarantine) before offering it for sale to make sure that you're buying a healthy fish. Maybe you can talk to your LFS about such an arrangement where you pay for the fish, and ask them hold it for you for a week; it may be free or it may cost a little extra. They are more likely to do this service for an expensive fish (gold nugget pleco) than a cheap one (common pleco).
In most tanks that are sized properly for a pelco adding in the store water should be no issue. Anything in the alive in the water is also on the pleco, and anything chemical in the water didnt hurt the pleco in a tank with that consentration so diluting it extreamly but pouring it in is no issue.
Other than that i have no real idea how your plecos keep dieing.
i have been buying/collecting plecos for a while.I have over 100.From my experiences i would say your ph is too high for starters.Gold nuggets are a fragile pleco requiring pristine conditions.Also check the eyes of a pleco when purchasing.They should not be sunk in at all.The best luck i have had is buying plecos that i have been watching for weeks at the store.Shark aqurium in NJ is a great place for buying plecos.They are fed well and you can usually see them eating zuchinni so you know they are eating.
I would talk, have words, w/the LFS and see if they QT any new arrivals. Seeing you have lost more than one in a few weeks, I would look at the LFS' husbandry skills.