you can use whatever yeast you want, but just be aware of the difference it will make in the flavor of the finished product.
ale yeast should leave some sweetness behind.
champagne yeast will ferment dry.
As for how much to add.... i wouldn't worry about adding too much. Whatever you add is going to multiply and reach optimum levels, so just blast it with whatever is most convenient.
As a test, I made (or am making) a 1 gallon batch of cider. I just used dried ale yeast and tossed in one packet (6 grams, I think.) This is 1/2 the amount I would use for a 5 gallon batch of ale. It was probably overkill, but I didn't see any point in saving half a packet of dried yeast that only cost $0.75 in the first place, so I just tossed it all in (no starter, no rehydrating... just sprinkled the dry stuff into the glass jug and sealed it up with my airlock.)
Ground Fault Brewing Co.