Max size 2 cm shell length
1 gallon per snail
IMO you should get pest snails first allow a colony to become established and then add the assassin snails. Pest snails are only pests if you over feed your fish and are quite colourful and interesting to watch aswell as offering a buffet for your assassins. They can be substituted with catfish pellets,carrion and some flake foods and will prefer some catfish pellets over live food however why do that when it is so easy to create a natural food source for them and its fun to watch them hunt.
They prefer a sandy substrate
Apple snails are well protected they have a trapdoor to their shell which they can close when threatened and are much larger then assassins. They are still at risk though, I have seen 3 of my assassins attack a pouch snail that had a shell size of 4cm but the attack appeared unplanned and simplt because it wander past when 3 of my assassins were hunting. I do keep apple and nerite snails with my assassins and none of those have been attacked so far (2-3 months).