I've never had a mystery snail, but I think they're a beautiful species with a lot of awesome variety. I'm sorry to hear about Gary, it's never fun having a pet pass after having them for so long.
I considered ghost shrimp, but after reading so many people say that packs of ghost shrimp will take down fish, I started looking at amano (yamato, or japonica) shrimp. I don't like that they're more often then not wild shrimp, due to the difficulty in breeding them, but they are a larger variety of shrimp that will tackle algae and detritus voraciously. Whichever you choose, make sure you have some good hiding spots for them, being near larger fish will stress shrimp out, prompting them to hide as a natural reaction. A quick word about the amano shrimp, they will eat dying and dead plant matter, some people say they have eaten whole plants, but I would say that is due to not allowing the plants enough time to bounce back after being introduced to the aquarium, or they had unhealthy plants and didn't realize. So if you do get that kind, just let your plants mature for a month or 2 first or until they have steady new growth.