The bags have a metallised barrier layer that keeps most o2 out. The opening is a 2" fitting that gives an excellent seal so not much air gets in that way. The bags are good for at least 6 months storage, you probably want to bottle after that anyway. The disadvantages of bags are that you can't rack, you have to leave the cider on the lees. From what I have read, 6 months on lees is fine for cider but some people like repeated rackings to clear their cider. Also the bags can get co2 between the layers so you shouldn't fill them too much, I leave room for some big bubbles forming between the layers.
Quantities of yeast etc are easy for larger volumes because the big packets of yeast come with full instructions.
This is the instructions off a packet of cider yeast, Maurivin AWRI350