It sounds as though you already have a good knowledge of how water chemistry works.
In basic terms, KH (carbonate hardness) affects how much the pH can change within the water. The presence of the carbonic acid in the water stops the pH from fluctuating wildly when it is influenced by CO2 or by other pH influencers.
You'll probably be familiar with the classic advice given when attempting to alter your pH: Chances are, you will need to lower your KH so that the changes to the pH have more impact on the actual pH of the tank.
If your KH is 0 that means your pH can be very easily influenced by any factors that would affect your pH. In reference directly to CO2 check out this picture from practical fish keeping:
You'll see that the mg of CO2 per litre that you need for the pH to fall to 6 at a KH of 0.5 is only 15mg compared to the 87mg you might need for the same pH at a KH of 3.0.
'Okay fine, but what's the answer to my question?'
Injecting a small amount of CO2 into your water will have a big affect on the pH due to the low KH. This may not actually be a problem if you have very limited sources of pH influencers but there is a risk.
I don't know about crabs, but generally very low KH levels are not advised with fish because the pH can swing wildly as a result of pH influences. These can be substrate, decorations and many others. If your environment is very stable (free from potential pH altering sources) carefully injecting CO2 may be feasible for you but low KH and CO2 injection always make me a little nervous.