Some questions about rooting and flashing on the one m9!!

OK guys just a few question's here about the HTC one m9. I've always bought nexus phones since the nexus s. I've been him hawing around about buying a one m9 to scratch my cell phone itch to keep me till the next nexus release. So my questions are, I'll be buying the T-Mobile variant, is it possible to flash the international variant ROMs on the T-Mobile variant? Next, do I have to have s-off to flash custom kernels and Roms? From what I'm gathering I don't think so. I have adb and fastboot already set up on my laptop as I've been flashing ROMs for a long time now. Granite it's way easier on a nexus lol. Thanks in advance:good: oh and one more thing, is the bootloader already unlocked with the T-Mobile variant?