Five gallons just isn't suitable for any amphibians excepts for larvae and juveniles in my opinion. I keep some fire salamanders in a one gallon tub, but this is terrestrial ( no water troubles ) and is only temporary.
The smallest recommended tank size for aquatic amphibians is 10 gallons. It would be
possible I suppose to keep M. alpestris ( Alpine newts ) in a 5 gallon tank if you only wanted one. But I really wouldn't advise it. It would be quite cramped and would be quite hard to maintain the water quality and temperature.
I personally would do a 20 gallon, because then you could have 4 or 5 and it would be much easier to maintain water quality and temperature, give them more room, and provide a good planted tank.
Here is a great care sheet for this species.
Caudata Culture Species Entry - Mesotriton (Triturus) alpestris
Other species that you could
maybe keep in a 5 gallon tank include H. orientalis, C. cyanurus, and pretty much most juveniles species of newt and salamander, until they got bigger of course. But like I said, I wouldn't recommend it. And also terrestrial is easier to maintain in small tanks that aquatic.
Hope this helps. -Seth