Depending on the paint you’re using Xylene (Xylol) may be as much solvent as it is thinner. Depending on what thinner you’re comparing it to it may be slower drying. If that’s the case I’d guess it will give you some additional leveling and gloss compared to a faster thinner. Xylene will have no direct effect on the chemistry of the paint only that which might come from changing the drying time.
Mar-Hyde 2612 is an isocyanate based hardener. I hope you had respiratory protection.
I’m not familiar with the chemistry of the acrylic enamels. You can get hardeners made specifically for the acrylic enamels although you may have to go to a paint store which sells industrial finishes.
There are hardeners made for the oil based paints that need oxygen to dry. Those are not isocyanate based hardeners.
Is everyone talking about the same Rustoleum paint? Based on the reaction you had I assumed you were using the alkyd-oil Rustoleum. However Rustoleum makes different kinds of paints including urethane based enamels and they also sell accelerators for their some of their paints. Some of their paints will probably cure with this hardener and some probably won’t
