In a 25g you should stick to 1 species. They are small (most of them) but they can be aggressive, depending on the species. Breeding habits are different between species too, which will decide how many you could have.
Multies (Neolamprologus multifasciatus)
would be your choice if you want more than a couple pairs of fish. They live in colonies, don't eat their fry, and their numbers will continue to multiply.
You could start with 5-6 (try to get 1-2 males and the rest females) and soon they will breed and have fry which will add more to the tank.
Skip the rock and just buy a ton of shells. They may hide behind rocks instead of using their shells and you won't be able to see them much! Plus when they are in and out of shells, they are super cute.
I'm not sure if a Bristlenose would eat fry. Personally I wouldn't keep it with the shellies.
You could use regular pool filter sand with them. What is your water hardness and pH like? You may not need coral sand. You don't need an airstone or pump....just a filter with a sponge over the intake (so fry won't get sucked up).
If you dont like Multies for some reason, there are other options. Brevis
are similar in size and care, but they pair up, so I'd only suggest 2 pair in a 25g...maybe 3. Ocellatus
are pretty, but they are aggressive, so you'd be lucky to have a single pair live in a 25g without killing one another.
FWIW, my Multies are very friendly. They beg in the front of the glass, follow my finger down the glass, and are almost always out of their shells. It took mine a good couple months to settle in though. I have probably 6 dozen shells in a 29g (or possibly more).
I suggest starting with atleast 2-3 shells per fish...adding more as their numbers grow.