assuming full tilts fit you, then you'll be fine in a 6 flex for sure. i'm about 135 pounds, i have no trouble flexing my drop kicks with a 6 flex tongue. you might notice a flex change at different temperatures though. on warm spring days they are super flexy, but in the cold winter they felt slightly stiffer. you will not have a problem.
and yes, be careful with the buckles. but if you really aren't worried, buy hot doggers and ride them until buckles break. i've had ft plastic buckles all year and they are still intact. then, if you do break them, buy some metal ones and replace.