Shelldwellers are very resilient and actually like a high flow rate/water movement as the lake they are from is tidal. I would steer clear of the mollies as if they try and enter the colonies territory every adult will mob them.., I would just commit to the shellies and fill the tank half full with escargot shells (east to get online from any french delicatessen site) and you will be very happy. I kept multies but brevis could work just as well.. just do your research and get the species you want and yo will be very happy. I loved my Shelly tank and having written this and seriously considering setting up the 100ltr I have in the garage for another! Lol