Can we use a gas cylinder made of A6061, meant to carry air up to 300 bar, with Hydrogen up to the same pressure? If not, what is the maximum pressure that one can safely use hydrogen with?
Structurally the pressure is independent from the nature of the gas. So, it shouldn't matter if its $N_2$ or $O_2$. Special care should be taken with corrosive gases, but usually that means the containers need to be manufactured with corrosion resistant materials.
Regarding $H_2$ my main concern would be if it leaks out. Hydrogen is the lightest known gas. It may leak out of systems that are airtight for other gases and may collect in poorly ventilated upper reaches of buildings.