The cloudyness from the sand will settle after a couple of days, you should see improvement within 24 hrs though.
not MLS, its MTS = Malaysian Trumpet Snails, most pet stores will give em to you for free as they find them to be more annoying than beneficial. Gouramis are great fish for the top but they come in a large variety of sizes and temperaments. Almost all are territorial but some, like the pearl, will be more agreeable and less aggressive for its own kind.
I have 2 Bolivian rams myself, they are wonderful fish to keep. Mine are very aggressive towards eachother and have very defined territories.
For schooling fish, rasboras are very good at schooling. Many of the smaller tetras are also very good for creating a school.
Corys are amazing. I used to keep some dwarf ones. They will school in groups of 6 or more and will swim together. They also love sand, it will be perfect for them.
The Tropical Fish Profiles on this forum are very good and well researched resources. Most of the fish that can be commonly purchased in stores are listed with care requirements, space, etc.
I would recommend getting a test kit to make sure your water parameters are suitable for the types of fish you want.
I also recommend that you introduce your stock in little batches of no more than 4-6 at one time. It will limit the strain on your bio-load and will let you monitor fish behavior and compatibility before adding more.
As for plant amount, think of what kinds of plants you want and just plant them in places you like. if you notice that you are getting a lot of ammonia, you can add more plants and do a small 10-15% water change. Try introducing the hardier fish first as to minimize fish loss.