5 gallon tanks are hard to come by. Do you already have one? There's little point in purchasing one tank that's already going to be expensive, just to buy another tank.
And I wholeheartedly agree with Francois. You should never mix species, and you should especially never mix any species with 'red spotted newts', Notopthalmus viridescens. They're extremely toxic.
If you are taking your own species from the wild, be sure you are doing it in an ecologically responsible manner. This means harvesting only juveniles or eggs from large, healthy populations, and only from non-protected species. And only harvesting when and where it is legal to do so. This includes having all of the appropriate permits to harvest salamanders from the wild.