There are only two safety factors:
Pressure. Output of one element must be in safe range of another element's input. If it isn't, you can use pressure reducer.
Fittings. They must, well, fit. It's not enough for them to somehow snap, they have to work with each other by design and by technical papers. And must be designed to work at the pressure, see point 1.
Gas proportion is a matter of personal taste, not safety. I drank beer on which pure CO2 was used. Nothing special in it. A bit more gas in beer per psi unit, probably. Nothing that would affect safety.
And as far as I have seen, "food grade" CO2 came from the very same tanks "regular" CO2 did. Go figure. I believe it's simply cheaper to have one better grade installation with higher output, than two different grades with lower output. Ask your vendor, both in official and unofficial way to see if that's the case for you.
If pressure is too low
Sometimes you might just skip reducer or use different one. As far as I remember you hardly ever need more than 30 psi, so 57 Bar / 530 psi bottle should be enough for this purpose - if your pressure reducer can work with it. Some of them have not only upper, but also lower limit on input.