It would be a good idea to get your water tested for GH and KH, that way at least you'll know if your water is within recommended for shrimp. The only issue will be if it is on the extremes, as in very soft or very hard water. If you're getting the shrimp from a local breeder who keeps it in local tap water, then you're likely home free.
For the Aquasoil, the folks who soaked it just soaked it for a week, and did daily almost 100% water changes IIRC. There's actually a thread on it.
Found it, see last post
Huh, I lied, it was one person, not several who mentioned soaking the Aquasoil.
No issues with anaerobic particles unless you're going very deep (4" or more from what I have read), which you can't in that size tank, ha ha, more than a third of its height, sure you can make it that deep but do you want to???
Osmocote is ok for shrimp, within recommended amounts. I'd probably only put a quarter to a half teaspoon for that size tank. Max.
Fwiw, I used Aquasoil in a 12 inch cube, didn't use it for the whole bottom, two inches in the back and one inch in front, about 3/4 of the tank. Didn't leach much ammonia, after all I had read about it I expected high numbers. I did a 70% water change the day after filling, before testing lol, then it never went beyond 0.5 ppm. I actually had to add ammonia to cycle the tank (had to look through my notes for this info, but it's there, I knew I kept notes for a reason