gouramis do no school, they are like bettas, they will fight if there are too many. They also enjoy the mid-top region of the tanks. With 100 gallon you can keep bigger gouramis, they get very large, for blues or golds, you might be looking at 3 max
For substrate, any kind of gravel will work, however, substrates like flourite, eco-complete, and aqua-soil are specialized for plants and are high in clay so they slowly release minerals for ur plants saving u from having to dose ferts for the first few months. They will also help your plants establish themselves faster. If you are using normal gravel, the smaller the grain the better. Sand works well if you want it, but you need to turn it manually or get malaysian trumpet snails to turn it for you. Lastly, not for beginners, but soil will also work but requires research and is very messy/difficult to work with.
For 100g, you can fit a lot of fish. You can use the general rule of 1inch per gallon of water. However, some fish produce larger bio-loads such as plecos and goldfish (not compatible with gouramis), and some produce less like neon tetras.
With your setup, a few gouramis for the top will be ok, and for the mid/lower portions, you can think about schooling fish like rasboras, tetras, etc. Rainbows will do ok, some species will not school, others will, threadfinns (personally my favorite rainbow fish) will school shoals of 6 or more are very attractive although if you have too many males they will show a little aggression towards eachother. For your bottom, commonly kept fish are cory cats, theres a huge variety to choose from and they group up and swim very actively, if you have sand substrate, they will dig in it.