Universal
They use the same ASA design to screw fit cylinders
Hoppers are also the same, some feednecks may be slightly tighter and it could have been necessary in the old days to sand a hoppers feedneck
The exceptions to the rule would be pistols that use 12 gram co2 cartridges and the tippman c3 which used propane
Other than those you have 3 general choices with cylinders:
Co2 - don't bother unless you can only use co2 at sites you attend, and this would restrict your choice of gun as though the bottle will fit, co2 will damage most electric guns
3000psi air - standard choice and best value for money
4500psi air - more varied choices in shapes and sizes