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I'd find out what they were used for first. Next is if they are capable of being pressurized or not. Pressurizing a tank not made for pressure can be deadly. Be very careful who knows what they've been used for prior to you being there.
I think there are different Gas bottles for different types of gas. For example, you can get Co2 for soft drink despensers which just has gas come out the tap, because the Co2 in the bottom of the tank is liqued and the gas at the top is gas.
But you can have Co2 bottles with a metal "Stem" in it that draws liqued Co2 of the bottom. Then you have fork lift LPG bottles that are designed to work on there sides etc etc etc.
I'd be iclined to see if there are any markings on the bottles and do a Google search
all that matters is the nozzel, you can convert almost any tank meant for holding gas to hold any other gas i.e. i have converted a co2 tank to a nitrous tank and vice versa. they just have to remove the nozzel and put the one on there for that gas. I beleive they make the nozzels diffrent sizes as a saftey feature so you cant have a tank labled co2 with nitrous in it etc etc. it just takes a few minutes of welding. most places do it. Or they will take the tanks you have as trade for a co2 tank so long as they are in good condtion.
The tall/short combo you're describing sounds like an oxygen/acetalyne set up for welding,especially if the big tank is green and the short one is black,the tanks should be marked though.