First off, is there a way to feed them something other than crickets?
Second, do I need a humidifier if their tank water is heated, what should that temp be?
Third, will they destroy my RCS (don't care if they eat a few, I'm worried they will exterminate)
lastly, how much land space would be needed, (I'm thinking around 1/3 land)
any tips/suggestions would be great!
1) IME they will only eat something if it is moving fairly rapidly. Frogs eyes are designed to capture movement and (according to sources) can only see outlines of shapes so will not try to catch something that isn't moving. Some foods include crickets, active mealworms, earthworms, flightless fruit flies etc.
2) They do not need a humidifier in their tank. As long as there is some water movement (Small filter or sponge filter) the humidity should be fine. FBS are native to East Russia, North China, North Korea and South Korea. There have been some cases of them popping up on Japan but that is most likely due to people releasing them. They do experience freezing winters and fairly warm summers. IME they do best with some change in "seasons" but if that isn't possible the temp can be kept constant between 55 and 80 all year around.
3) This depends on the depth of the water and the cover that the RCS have. In deeper water, the frogs generally do not go to the bottom but if the water is 2-3 inches all over, they could eat quite a few. Just provide plenty of cover for the shrimp and they should be fine.
4) Half and half is better. The main thing to consider is that they spend a ton of time on the shore so if you can set the tank up with the "beach" parallel to the front glass that would make for the most shoreline (Diagonal is even better if possible).