Hi
Well here's my two cents. lol A terrarium by nature tends to raise the humidity and hold it. So the usual problem is too much. another is the air is stagnant. Another is that light levels are too low
Another is that the atmosphere is Paradise for fungi lol
All these problems can be handled in fact many types of plants have no problem at all. Many others croak.
Obviously the choice of soil would depend on what type of plant just like growing in a pot. In fact many terrariums keep the pots to have various soil mixes, ease of removal and so on. Many types of tropical plants will grow without soil at all. because the humidity is kept high. Most types of soils sour very quickly,again due to high humidity, poor drainage. So any mix would depend on your choices of plants.??
There is a gage called a Hydrometer which measures humidity. Available in any deparment store .Many also have thermometers also. Come in many sizes shapes accuracy and cost lol.
Generally most flowering plants require more light than provided by a standard aquarium bulb but you might find some exceptions. Bear in mind you'll have very limited grow space so many plants would quickly get too big.
For your first I'd suggest a "warm tropical" limited to ferns and maybe some mosses, until you get the hang of the management. gary

I have had some luck with terrariums. The most important factor is light. If your terrarium cannot be located by a window you will need a "plant" type bulb for success. You may be able to find one that will fit your aquarium fixture at the pet store, if not they are available to purchase separately. The flourescent ones work best for a small aquarium as the bulb types can create to much heat.
Small ferns, bromeliads, even some miniature orchids will work but you should do the research on each plant as some will have to high a light requirement to do well.
I have used some carnivorous plants with success but always keep in mind that you will not be able to provide "full" sun.
You can add a stick on heating pad if the temp. gets to low in your house. And you will need a temp./humidity gauge, you can buy them cheaply at the pet store or online.

Regular Cool Whites work just as well or better than plant bulbs. Anything from 4100k to 6500k will work for overall plant growth. T12s and T8s work but T5s work even better. I'd go without the gel. Use a layer of charcoal on the bottom and mix MG potting soil with perlite (rinse to remove dust) and/or sand, 2-1 or 2-1-1 for your soil. Let the top of the soil go dry before adding water each time you water(Just a guideline, don't let plants wilt). This will ensure that the water does not stagnate and also cut down on the chances of developing mold, mildew and fungus. You may also want to use distilled water. If not salts and minerals will build up on the surface giving it an ugly white film after time. I use an aquarium air hose and quart jar (siphon) to water to minimize surface disturbance and keep water off the leaves. This also helps keep the mold, mildew and fungus to a minimum.