Charging stations coordinate with the vehicle to determine the charge rate, so there's no restriction based on power. Other EVs technically could use the Supercharger network, though many of them wouldn't be able to charge at the SC's maximum rate. Tesla has decided to keep it exclusive for now.

There are actually other charging networks that have more powerful chargers than Tesla; for example, while the V3 SCs max out at 250 kW, Electrify America's most powerful chargers max out at 350 kW.