1) Cycling is an excellent aerobic exercise. You said cardio, but I suspect you meant aerobic.
2) Ride as often as you like. I'd give the legs a day off after heavy lifting, but that's about it. Make sure you give the bod at least one day off a week.
3) Riding should help tone things up, at least a little.
4) If you are weight training, and riding, your muscles are already fit and toned. What you need to do is eat a good diet. That means lots of fruits and veggies, and not junk food. Strip away a little fat, and reveal what is already there.