PT, pressure treated lumber from the yard is still very wet. If you knock it with a hammer you will likely see liquid ooze from the trama area. Drying out really depends on the region and the time of year.
In my area, Texas, with over 100 degree days in the summer, a new deck will dry out in 90 days for sure. Then it can be stained.
I tell my customers that think of the new lumber as a sponge filled with water, to add more water to the sponge, you have to release some of the old. Same with the lumber, if you stain it now, the liquids have to leach out and with that leaching, the stains and oils go with it. So, the wood has to be dry first.
Now the question on railing height, this is another regional and code thing. In my area, a railing has to be 42 inches high with spindles every 4 inches. This is only for a deck that is off the ground. A flat deck on the ground in my region, does not require railings. So, check your code in your respective area.