HIIT cardio is very effective, but it can also burn you out faster than you'd expect. It is very easy for your legs to become overtrained if you follow through with an intense HIIT/bodybuilding regimen at the same time. This nearly happened to me during my last cutting up cycle, so I had to cut it short.
I would advise you to REALLY start off slow, maybe with 5 minutes or so, and gradually build up to 15 minutes, tops. You may think that 5 minutes sounds like a waste of time, but you'd be surprised at how effectively the fat is burned even in such a short time.