There is no specific answer to your questions, because everyone reacts differently.
1. HIIT generally should not be done every day. After a while, it can be hard on your system and you can give up. I generally rotate between 3 types of intervals- walking, jogging, and sprinting (HIIT). However, you should try it out to see what you can tolerate.
2. On the off-HIIT days, do whatever cardio you want- gerbil (endurance) or a more moderate interval training, or a different machine. They each have their benefits.
If you are starting weight training, you may find that doing HIIT the day before or after leg day may be a bit much. Try it out and see how it goes.
Do or don't do; there is no try (Yoda)