nails u can pound in but screws unless you want a lot of split wood you have to pre drill the holes
it will only crack with really old dried out would..if u pick up wood at the shop it should be saturated at 19% moisture so if u build it when u pick it up you should be fine
The truth of the matter is that if you have to ask the question, you probably don't have the necessary skills or knowledge to build what your building safely.
2x6? for the deck of it...thats a alot of unneed money spent...you could just strip pallet tops which is free or use 1 by 5 decking boards that is pressure treated and meant for outdoor use.