You've asked about htis jack in another thread and were given the tools necessary to figure this out... but even better yet... was the suggestion that you consider other options. With as many questions that you have on this it has me thinking that maybe you shouldn't be building this thing.
But... I expect this is a homework assignment and the board policy here is not to do homework for people.
For what you are asking you will need to consider all of the questions you mentioned and some more. You might have a restricion on the size of the unit or work envelope. Then you need to know what this jack needs to do? Lift what? How high? Any ground considerations? 500 tons isn't a toy... that's some serious jack you're talking about.
Also maybe consider using a few smaller jacks working together to get the job done? Maybe a bladder if this thing you're lifting would be suited for that? Take a 100" x 100" Bag that'll handle 100 psi and there's your 500 tons of lifting capacity for considerably less $$ I'd figure.