One mystery snail is probably the maximum. You can add the shrimp if you get a nerite. Just make sure you're on top of cleaning. Monitor the levels at first to make sure your BB can handle the new addition.
I'm not too worried about the levels, since it's been cycled for a while now (plus the soil releases large amounts of ammonia, so I think it's a steady cycle, lol)...but I will keep an eye on them. Thanks again.
Current bettas: ♂Opalo, ♂Granite, ♂Alpha-Betta, ♂Eclipse, ♂Lucky, ♂Honor, ♀Virtue, ♀Innocence, ♀Garnet, ♀Chroma, ♀Tethys, ♀Amaryllis, ♀Goldeen
Tanks: 3g betta NPT, 20g NPT community, 10g 3-way divided, 10g NPT sorority, 5g divided betta tank