In my last reply, I actually did recommend a scoop of whey, in responding to your question about sipping something after weights but before cardio, I said use either "water or a protein-only drink".
The protein-drink would be water with some whey powder mixed in, albeit a small amount. You only need a small dose of amino acids to stave off catabolism of your muscles in the short term.
I am only strongly recommending you stay away from ingesting simple carbs before any cardio you're doing with the hopes of fat loss. You had asked about sipping on Gatorade, which is a bad idea.
I hear you about being mortified of losing hard-earned muscle mass. But if you're eating enough quality calories throughout the day, the 20 minutes of HIIT after your weight training really shouldn't have a negative effect on your muscle mass at all.