Hi there,

If you did not stock allready there's little lost.
Just go ahead as you described. Don't worry too much if * some * of the
calcerous gravel stays in the tank (just be sure not to toss out too much
flourite)since your tap water is very soft, so a little additional hardness
added by the gravel wouldn't hurt that much. Even the GH10 you have now
wouldn't be that bad, if it was a constant factor (which it's not now... for
obvious reasons).
I have no experiences with flourite, but I read good stories about it, you
could use it as a sole substrate.

I would be carefull with the addition of CO2 since water of KH2 might be
prone to PH crashes.