1. Black Diamond will work well. Locally Tractor Supply sells the medium and fine size particles. Almost everyone who buys for aquarium uses gets the largest particle size they can get, so you probably have medium. If you are concerned, it is only about $9 for 50 lb.
2. Your plant package should come with a list of species. If so, it is pretty simple to look for photos on Plant Finder, and match them up with the plants you receive. If you are stumped, post a photo in the Plant ID forum. One caution: a full size Amazon sword will quickly outgrow at 20 long, so if you have a choice, get one of the smaller species or cultivars.
3. If you want to create a significant slope build up the high portion first with something solid and inert. Ceramic tile or flagstone work well. I like ceramic tile because yoiu can easily break it into pieces and stack in layers to get the shape you want. Don't make the slope too steep or you have have problems keeping the cap and soil in place.
4. I am fairly familiar with the Finnex fixtures, and a 24/7 over a 20 long tank will give you too much light for a Walstad tank. I use a Finnex Stingray over a 20 long, and that is plenty. Does the 24/7 allow you to dim the light? If so, start out at 50%. Otherwise, raise the fixture or use screen and/or floating plants to reduce light.
5. The main purpose of a filter in a Walstad tank is to provide circulation. This distributes nutrients, CO2, and O2 evenly throughout the tank. A powerhead or simple submersible pump will do the same. I use filters on my tanks so that I can put mechanical filtration medium in the filter to remove particles from the water. Normally I do not use any chemical or biological media unless there is a specific problem with a particular tank. Sometimes I just run the filters with no media at all.
Plants do an excellent job of removing ammonia and reducing flow if you have enough of them. In a well planted tank, a betta can always find a quiet spot to hang out.
Do you really need a heater? Unless your home is unusually cool at night you probably don't. I don't have heaters in any of my tanks. For an excellent article on the subject, go here: http://www.seriouslyfish.com/whaddaya-mean-too-hot/
Good luck! On 28 April is the next meeting of our local club. More information here: https://www.aquaticplantcentral.com/...ing-april.html