personally i wouldn't do ghost shrimp. dunno if he'd wind up with brackish shrimp (some say there's an identical brackish species, i dunno) and those buggers go hunting IME. likely to get one of the guppies at night. there's also the likelyhood of mass die-offs like otos. that's my experience and understanding. i do know that amanos and otos long outlived my ghost shrimp back in the day, though.
i probably wouldn't do "apple" snails either. they're big waste machines and can get pretty large. not good for someone having a hard time wanting extra species without overstocking a small tank all while being a noob.
amano shrimp... sure. red cherries... you bet... but i wouldn't expect their numbers to grow. tiger shrimp... why not. sulawesi shrimp and snails... yeah... i'd do that. ramshorn snails... if they're colored... and only of one color... absolutely! blue, red, orange, whatever... pick a color, or get what you can get in the marketplace here that's not brown. trumpet snails... you betcha! rudolph shrimp... so cool... awesome antics... same "glass" body, but this freakishly cool red rostrum.
plus... everything i listed should be fairly to extremely low on the bio load (so if they say reproduce to dozens, it's not going to hurt anything), mostly all could be considered part of a "cleanup crew" and should be fairly easy to care for. (as always... definitely be sure to double check exact species information.) i will reiterate, though that checking msj, mg and the marketplace here will net you stellar stock at a phenomenal price in most cases.