HIIT should only be done a couple times a week because of its intensity. HIIT is used so you can squeeze the most fat burning potential out of a short cardio session which prevents you from having to spend long periods of time in the gym. In my opinion, if you are doing HIIT everyday you are really spending too much time performing this type of training. I am just guessing here, but I think if you are doing HIIT every morning you really can't give it your all cause your system is getting burnt out. And if your HIIT sessions aren't hard then you probally aren't going all out like you should be. Maybe you can do it this much but I have really never seen anyone else, even advanced trainers, recommend this many HIIT sessions a week.
If I had a nice track near by I would do sprints over using a bike. HIIT is all about preferences however, so a bike will work just fine as long as you go all out. I prefer a track over treadmills as its easy to get up to your full your speed and stop on a dime, unlike a treadmill where you have to play with settings. That being said there is nothing wrong with a stationary bike I would just make sure that it has built in resistance that you can adjust each week. One thing to keep in mind with the bike is that you are not getting an upper body workout, which is why I lean more towards an eliptical when doing HIIT on a machine. Might as well maximize your time right?

If you are doing HIIT 7 days a week PLUS some sort of low intensity cardio 3-4 times on top of that I would say you are overdoing the cardio. I would cut back the HIIT to 3-4 sessions MAX. You are running the risk of losing alot of muscle mass by hitting the cardio so hard. Just remember, it is alot easier to lose fat than it is to put on muscle so you really want to be careful.