Mine are with Tangerine Tiger shrimp, #1 best tank mates seem to be recommended as Red Cherry Shrimp. Plus I have a couple Anchor Cats aka Moth Cats, and as fish I have the diminutive Boraras Brigitae, aka Chili Rasbora, one of the smallest nano fish generally found in the trade (though there are a couple smaller).
Their mouths are very small and they do not seem to "hunt" microfoods like some fish enjoy doing. I have always seen them prefer to pick their food from the water column. I feed them a few things but one is Micro Pellets, and they need to be reduced in size for young fish usually found in the lfs
Smashed a little more with the back of a spoon say if you wanted to do the food all at once. Or I just grab it in a pinch with the thumb and forefinger and rub it back and forth further reducing the size into the water.
They are in a 6.6G Fluval Edge tank. It is well matured, stable filter media and I would really recommend this as a 10G tank set up over the smaller one. I do frequent smaller pwc
TMC like to be in groups of around 6 say most sources. They literally hang out at the top of your tank mostly, if you have floating plants like Red Root Floater, Salvinia Minima, or Frogbit. They will cling to the roots and eat microscopic particles of food, critters known as aufwuchs. I also have added a few floating Wisteria plants too.
What size tank are you thinking of using?
The tiny crabs could be in a smaller tank but it would need to be a very stable and mature tank, perhaps without the fish, just TMC and Cherry Shrimp. Tank stability is critical or death will be on the doorstep. Smaller tank, larger danger zone.
They would be awesome in a location where it is easy for you to see right into the tank. Near bedside, on a desk, next to your favorite chair hangout area. Since these nano creatures are so very small you would maybe never see them otherwise. Besides, watching shrimp wander the tanks is an entertaining activity. I would add a couple Ramshorn snails of a fancy color, also fun to watch.
Sand substrate, a little heater sized for your tank and a little filter I use many AquaClear smallest one for the Edge tank.