Low pressure is considered to be around 450 psi. High pressure is around 850 psi. If you have a ninja tank you can adjust the output pressure by removing shims inside the regulator. Just read the manual.
Most guns work just fine with a standard output tank, some guns without macroline suggest low pressure, like the empire axe, although the axe runs just fine with a normal tank. If a tank doesn't specifically say what the output is assume it's normal output.
Unless you plan to buy an angel or some of the bob long guns, you do not need to match tanks to guns.