Yeah, you don't need shrimps or snails (but when you buy plants a lot of snails tend to hitchhike, so you'll get them regardless).
A snail would work in the divided 7.4g, maybe a Nerite snail, good algae eater, eggs cannot be hatched in freshwater, stays relatively small. As for shrimps the cheapest shrimp I can think of are ghost shrimps, they grow quite large for shrimps, around 2" or so. Scavengers, they eat anything that gets left behind. Drop some extra pellets or get some sinking shrimp pellets for them. :) Maybe 3 ~ 4 of them would be good.
If you wanted to do a fancier shrimp I would recommend RCS, they are a dwarf shrimp, primarily known for algae eating they are actually omnivores, and scavenge like ghost shrimps. They're somewhat more expensive than ghost shrimps, and since they are so small they can be eaten by your Bettas, so there is a risk there.
But given enough coverage they should be fine.
When choosing a soil you want to find an organic potting soil. Organic is very important, don't want any of that silly additives in there. Cap it with some sand or gravel. Maybe 1.5" of soil & 1" of sand.
Measurements of the tank doesn't matter. Slow growing plants are anubias, java fern, marimo moss balls, crypts, and mosses. They grow very slowly. Unfortunately most carpeting plants require intensive light and CO2 in the tank, you could do a moss carpet. :)
My 6g isn't that great haha, just some water wisteria, java fern, anubias & moss laying here and there. Just wanted to get as many plants in there as I could.
In a 7.4g would could do some Endlers, maybe a trio or so? With only one Betta. :)
And I'm not sure about my lighting to be honest, my 2.5g is a clip on, and my 2g & 6g the lighting came with the tank.