You mentioned that the Ammonia is 0.
What about the Nitrite level?
Both need to be 0.
As
@Naughts and
@Essjay mentioned, if you didn't add any ammonia source during the cycling process, the beneficial bacteria couldn't grow.
What brand of bacteria did you use?
Anyway, assuming that your Ammonia and Nitrite is 0, the next thing that you have to watch out is chemical.
Did you use any chemical or detergent when washing your tank, filter, the bucket that you use to collect water, etc?
Shrimps are sensitive to any chemical.
By right crayfish should be more hardy than shrimps.
If your crayfish died, it means something is not right or quite serious with your tank.
I list down some basic questions:
1)Are you using any filters? What filters are you using?
2)Did you change the tanks water before adding your shrimps and crayfish (or after the cycling process)?
3)Did you use water conditioner for your tap water?
4)What is your tap water GH, pH?
5)Did you rinse the plants well before putting them into your tanks. Some plants might have toxic pesticides that are used to kill snails. This will kill shrimps and crayfish.
6)You may also want to check the fish store where you bought the shrimps and crayfish to ensure that they are kept in good conditions.
Anyway, Guppies are more hardy than shrimps but your tank must have enough beneficial bacteria to support them.
If not, you will have to test your water frequently, change the water whenever the ammonia & nitrite are higher than 0.
And feed them only once every 2-3 days if your tank do not have enough beneficial bacteria.
Also, don't clean your filter media for the first 1-1.5 month since the beneficial bacteria is still growing.
Your plants Java Ferns and Anubias are slow growing plants and won't be able to help much when you have more shrimps and fish.
Fast growing plants like Anacharis Elodea, Hornwort and Cabomba will be more useful to absorb the ammonia/ammonium in the water.