An 18” tall, low tech setup is going to have its limits & challenges mainly due to lighting if you want some form of carpeting plant tbh...
So lets break this down a bit and answer one Q. at a time...
1. Taller plants that could possibly work include Java Fern, larger anubias and some of the Crypts may do well once settled in...
2. For ground cover, since this is low light / low tech, i would consider a carpet of the smallest anubias species you could find and fill out the bottom that way... i’ve seen tanks filled with just different anubias that looked amazing using smaller anubias as ground cover and different larger species to fill other areas...
If you are super patient and have some money to spare, you could even consider some of the Bucephalandra plants but take note they are super super slow growing plants and can be quite finicky to work with at times... but the results when they work out are amazing...
3. Snails... if you dont have a good place to buy plants other than petco type places, i would suggest giving any new plants a quick 2-5min bleach dip in a 1:99 ratio bleach to water mix, followed by a soak in a seperate container with an overdose of decholonator mixed with water followed by one final good rinsing under the tap... then placing it in the tank... this should kill off any snails and possible algae before entering the tank...
4. Finally, shrimp... basiaclly any small ornamental shrimp will end up as an expensive snack for your fish, especially the chiclids... they only get a few CM long once mature, so unless you go for something like a bamboo shrimp thats a larger filter feeder, im not sure you have many choices... since things like Cherry Reds, CRS etc are all going to get eaten...