Hi and welcome to AA! Pics of the two crabs in question would definitely help. Honestly, I wouldn't recommend the crabs as an on-the-spot decision. It is good to do considerable research before setting up any SW
tank, especially if it's your first. SW
can be very difficult and complex if you don't prepare for your tank in advance.
Have you ever done a saltwater tank before? You'll need a tank larger than one gallon, in fact, the larger the tank the better. Temp and parameter swings are a major concern for small tanks.
In addition, you'll need to cycle the tank, which can take weeks as I'm sure you know. Live rock is almost a must, as it will house the BB that filter your tank biologically.
You mention that the crabs are brackish, SG
at 1.021, which is borderline full saltwater. In that case, assuming you use a conventional aquarium salt, you will need around 1/2 cup of salt per gallon, and then more and more salt for water changes.