It certainly is possible to overdose a tank, even with the Hagen, and certainly with a 2 liter bottle on a 5.5 gallon tank.
I've read of people who nearly OD'd a 10 gallon with a Hagen. You probably couldn't run it at the lowest setting with that small of a tank. It reallly depends on your water.
It is unwise to begin with CO2 until you know your starting pH and KH. Then you can establish what your target pH will be (about 0.8 to 1.0 lower generally), and the lower limit of pH will be, below that you will risk killing your fish. All this comes from the CO2 charts. And your water parameters.
So, what is your starting pH and KH?
FWIW, The hagen system is clean and neat and hangs on the back of the tank, and it fits the 5.5 gallon tank. All the needed parts are included. The unit does not leak, since it hangs on the tank it will not siphon water if you remove the top to refill. The mix is very good but you can run your own mix when it runs out. The diffuser never plugs up like an airstone can. After 4 airstones dissolved in my DIY setup due to the carbonic acids I gave up. The airstones also gummed up which required a bubble counter to scrub the goo out of the gas. All those connections leaked initially and were a bit of a chore to get sealed, even now I have to occasionally scrape the DIY set clean and reseal. The smaller tanks both have Hagen systems.