Japanese Phone/SIM (Docomo) - Safe to install custom ROM? (Galaxy S6)

I know how to root then flash my phone, but I'm in Japan and using a Japanese provider. The phones here are fairly locked down and I don't want to run the risk of using a custom ROM and being unable to use my provider.

Does anyone know if there are any problems with installing a custom ROM and using a Japanese provider?

I'm using a Galaxy S6 and I hate the Samsung default.