So sorry it took me so long to get here, please forgive. You mentioned that the tank will only be a few gallons? To begin with, I surely wouldn't keep 1 newt of any species in less than 10 gallons. Newts are dirty, they are EXTREMELY sensitive to water quality, and most grow quite large (6 - 8 inches full grown) Even the smallest species will need at least 10 gallons. The larger species will need a minimum of 30 gallons by the time they are grown, which, like fish, will happen quite fast.
With that said, the drawing you posted is the right idea for keeping newts. The things to watch for are:
enough space in the water to swim
enough land to stretch out on
enough plants to offer hiding places and shelter
Proper lighting overhead
Warm temperatures of mid 70's (fahrenheit) for the water (so you'll want to add an aquarium heater, must be submersible for a setup such as this
Filter to keep the water clean, run WITH carbon that gets replaced every 30 days, at least
Things to plan for with newts:
They grow quite large
They are VERY sensitive to water quality. Any levels of Ammonia and Nitrite will cause open sores. Nitrates, if higher than 40 for long periods of time, will cause the pH to drop, which will also cause open sores. These sores are prone to fungal and bacterial infections, very difficult to treat, and usually ends up to be lethal.
Newts are ESCAPE ARTISTS, make sure the top of the tank is WELL SEALED. Newts will crawl out of the tiniest holes, and can climb the glass pretty easily. If they get out, they hide and then dry out, thus they die. It doesn't take long...
Do a lot of water changes and gravel vacs, at least once/wk.
Watch the feeding, uneaten food will rot quickly and make a mess of your water quality. Most newts will thrive on live blackworms, (these are not bloodworms, there is a difference) and crickets, and the size of the crickets will make a difference and change as the newt grows. Crickets is one of the easier foods to give, it's cheap, you can learn to raise them yourself, or even order them in bulk for cheaper online. They're also easier to pull out if not eaten. If they die, just pick them out and throw them away.
Ok, I think I've about covered all I can think to tell you about newts... if you have specific questions beyond that, please, feel free to ask.