Welcome to the forum
First off, a ten gallon tank is the smaller advisable volume for the species. Mostly because a smaller volume makes it very difficult to maintain water quality. The species when healthy and adult is entirely aquatic. If housed properly they may never venture out of water, or only for a quick walk, so a piece of floating cork bark is more than enough for them.
The slope is a bad choice, as deep layers of gravel/sand become anaerobic, plus it takes a lot of the water volume. You´d be better off with a thin layer of substrate and the rest water...you should fill the tank to 3-4cm to the rim. The more water the better for your newt and for you.
About mixing, you can find several threads on this forum about that topic. Mixing caudate species is very unrecomended because there might be problems with predation, agression, territorialism, toxins, illnesses, stress....
Mixing with fish is rarely a good idea. Only a few small cool water species can be safely kept with newts. You can also find information about that in the forum.
Your newt should only be in the company of others of it´s species.
Since you are looking for recomendations, i´ll say that you could keep white cloud minnows with your newt. Also some small fresh water shrimps seem to do fine, although they may get eaten. Snails such as Physa or Planorbis often appear naturally in stablished tanks since they come with the plants, and they are fine.
An stablished healthy tank develops a whole community of invertebrates like Cyclops, nematodes, Planaria, ostracods, daphnia...which makes it full of life. Perdonally it´s all the variety and mixing i need in my tanks.
