Freshwater fish absorb water through their skin, while salt water fish literally drink the water then pass the salt through their pee. While a guppy can be kept in saltwater, and your right it has been done, it will dehydrate the fish if done improperly. Osmosis shock it's called. The salt will literally pull the moisture from the fish and kill it if done to fast. SO... To answer your question, in order to properly acclimate a freshwater fish, and don't fool yourself they are not brackish, they are freshwater, but in order to do it properly, it should be done over a course of a few weeks not a drip. A drip will adjust the fish to PH, temp, salinity and chemistry changes. A drip, no matter how slow cannot be done properly to change a fish from fresh to salt water.
Your best bet, if you really wanna go this route is to set up a 10 dollar, 10 gallon tank from walmart with freshwater and add salt over the course of a week so the fish can acclimate slowly and without stress.
Or you can throw in 10 and hope one or two make it. Then again, if your that deturmined to cycle with fish that are nonaggressive and can be eaten, buy a baby clown fish.