Yeah, I agree with Mark. You might have trouble with this project.
This is an idea that I have been pretty obsessed with over the years. Creating a self sustaining ecosystem is a goal that I have tried several times, and while i have not given up, i have learned several things . . .
Because you are starting with newts as the animal you want to keep, you need to think of the food chain required to sustain the newts, and then figure out how much space/nutrients will be required to sustain that food chain. There are all sorts of tiny invertebrates that the newts could eat but my newts eat several grams of food a week, and to have a population of invertebrates that reproduces fast enough to produce several grams per week, i would need a huge amount of plants/organic matter/smaller invertebrates to feed the population of whatever invertebrates the newts were eating. Luckily, firebellies (i'm assuming that's what you have) are pretty small newts, but I think they still eat quite a bit of food. You would probably need an indoor vivarium that had alot of floor surface area (height not necessary). I would estimate that something the size of a pool table might be sufficient (and believe me, once I buy my own house, there will be a newt pool table that comes into existence!!). you would also need to cram it with plants, and then add whatever invertebrates you choose to feed your newts with. letting the ecosystem grow for the bottom up and be healthy before adding newts to it.
Alternatively, an outdoor enclosure would be a self sustaining ecosystem that would rely on the ecosystem of your backyard to provide prey animals for your newts. this would be pretty easy, but would probably lack the sense of accomplishment of designing and growing a self sustaining ecosystem.
I would definitely encourage you to try what you can, but if space is a limiting factor, I think you will find it hard to maintain newts as your top of the food chain. perhaps you could use lissotriton since they are so tiny.
I am probably rambling now, but you have found my favorite topic in this hobby. If this self sustaining ecosystem seems as cool to you as it does to me, then I have some recommendations for you.
"Ecology of the Planted Aquarium" by Diana Walstad- This is the book that really got me thinking about how complex of an ecosystem an aquarium can be. It is very scientific, but it goes into detail about the flow of nutrients in an aquarium, the relationships between different species of plants, and all sorts of other considerations when setting up your own planted aquarium. Focuses specifically on fish, but most of the advice is transferable to planted newt aquariums.
Natural Aquariums - Guide to Planted Aquariums, Aquatic Plants, & Freshwater Invertebrates - this website has alot of good articles about setting up very low maintenance, nearly self sustaining aquariums. Again, it focuses on fish and plants more than anything, but I found it pretty useful for my newt aquariums as well.
Biotope in my study, a low-tech natural aquarium « Sustainable life, natural aquariums, science & philosophy @ tuncalik.com - this single blog entry has been a huge inspiration to me, but it just goes to show, how much space is required to maintain a self sufficient ecosystem. he basically does with fish, what you are trying to do with newts. I would recommend checking it out. And, since I never actually answered your question about what invertebrates might be good for a self sustaining ecosystem, he uses Mexican amphipods (Hyalella azteca), and Water louse (Asellus aquaticus) in his ecosystem and while I personally have never used them, I bet they would be good food for newts. I actually found an article that mentions newts preying on amphipods in nature. So maybe you could try them.
that was probably more of an answer than you were asking for, but I always tend to be pretty verbose when I decide to give advice. I hope it helps you at least a little.
