Well from my experiences with flourite, I had the best success with the regular flourite.
As for setting up an Iwagumi tank, I probably wouldn't do it in your case since you're new to the planted tank world. I would set up a regular planted tank, and go with that style until you get use to having a planted tank, and what the requirements of plants can en-tale.
The reason is a lot of Iwagumi tanks are done using ADA aquasoil, and some are done using the dirted tank method. Also Iwagumi setups require good substrates, co2, lighting, EI dosing, and with these it's best to start out with a regular tank setup until you gain the knowledge and experience with planted tanks.
As for doing slopes in a tank, most people tend to go with large rocks under the substrate, or with pantyhose stuffed with aquarium gravel as to give the substrate height. Then they cover them with their substrates and create the down sloping look.
15g Fluval Flex - Will be a planted shrimp tank