There is no size limit to the size of tippet material that you can use on a 6wgt. I have seen guys use 80 lb bit tippet in saltwater while using a 6wgt. So there is no true limit.
As for the floatant you are using it sounds to me like you have the old paste type? It is sticky and will sink your flies real fast if you use to much. Try using just a little bit at a time.
I would look into buy a better product such as Loon, Gink, Aquel or the old tried and true Mucilin.
I use, and have for many years, a product called Albolene Unscented hand cleaner. You can get it in the drug store and a jar sells for about 7 bucks and will last you a long time. Several season for sure. Just take some and put it into a smaller container and you are set to go. Just put a little on your fly and you are ready to go and this stuff will not gunk up your fly either. Ron