I don't think you need experience in writing het, if you've got a story you want to tell then go for it. It can be hard to move out of your writing comfort zone, when i first started writing fic, i kept away from het and femmeslash, because i found myself putting too much of myself into the female characters, and ending up with them feeling too personal. I'm trying to move past that now i'm a little more confident in my writing, and am hoping to write at least a couple of femmeslash and het fics for kink_bingo this year. I'll be interested in hearing how yours works out, i love reading about the way people write.