I suggest a couple of 8 cm fish only in a 4 ft. tank. Keep Tompots separate from invertebrates. Only the largest crabs will survive. They will still be vulnerable when moulting. Keep the Tompots well fed.
Boiled mussel is a staple.
Increase the food as the temperature rises. The Shanny , Lipophrys pholis can sometimes be kept in the same tank for short periods. Include barnacle covered rocks in the aquarium to keep the Tompots occupied. Tompots introduced into a tank full of Blennies, Lipophrys pholis , found themselves unable to compete for food in the long term. The ordinary Blennies were quicker on the uptake for boiled mussel, and crabs.