Is it completely enclosed? If so, I would think that the cacti might not take it well. Even if it is completely enclosed, you could probably get the cacti to work, but I think it would be more work than ferns and moss. I don't see any reason that the moss and ferns wouldn't work.
I am wondering about the "small light" that you mention, though. I know that some plants don't do well if they have light 24/7. I am wondering how big this light is. If it is bright enough, you could use it as the only illumination for the terrarium, but make sure to only have it on 12 or so hours per day (on a timer). If it is dimmer, though, you might need to put the terrarium by a window. But, make sure that the terrarium isn't in direct sun, as that will cook the plants.
A personal favorite plant of mine that would work is English Ivy. If it is used here, you will probably have to trim it every few months. You also might try Fittonia species.
-Jmcat

If you choose to grow moss, I've read it's important to get tropical mosses, as they will always be nice and green for your terrarium. If you go to your local nursery and take a look at their tropical plants section, almost any of those should be fine. As long as all of the plants you choose have similar water, light, space, and in some cases dormancy needs, you should be fine.

If your container doesn't have a lid then the cacti might be easier to maintain without having to worry about the watering as much. One advantage to using cacti is that there are many cacti which do not grow very fast, which eliminates the need for any trimming. Some cacti species I'm particularly fond of that are worth checking out are Astrophytums and Ariocarpus. They grow very slowly and have interesting and unique forms. Astrophytum asterias has many different cultivars with showy patterns such as the Superkabuto variety. With many smaller cacti, you'll see new growth each year but once you've had the terrarium for a while you can be proud to have nice, larger specimens.

I just recently ordered 3 tropical plants from the link at the end of my message. They, as well as other specialty vendors, have some neat specialty tropicals which could liven up your terrarium.

Here is a link that might be useful: T and C Terrariums

I would be hesistant to try a cacti or succulent. Many do not do well in stagnant air and higher than desirable humidity levels could be very problematic. But I could be wrong.

There are some small growing ferns that might work well and in addition to mosses, Selaginella could also be a good option

i am making a terrarium for a 4-h project what plants would work for all containers because i have not picked a container yet what plants or type of container would you suggest. it has to be under the size of 12" deep 18" long and 16" high please help me