Excessive amounts of cardio will hinder muscle gain, but as long as you are working out and eating enough to support muscle growth then you should be fine. Like mentioned above, you want to avoid a caloric deficit caused by excessive amounts of cardio as the extra activity will require more energy to support it (which means eating more). Since you should be eating more to support muscle growth the cardio is used as a way to control fat gain. The trick is to eat enough to support muscle growth and add in the cardio as a way of adding to your caloric expenditure. Its that fine balance of finding what your body needs to grow without becoming a huge land whale in the process

As far as what is the right amount of cardio to do, it all depends on your current calorie intake, activity level, and so on.

If your new to lifting and your simply trying to add some muscle and lose the fat, it will slowly happen as long as you train and eat right. Cardio is great way to burn a few extra hundred calories a day and its beneficial to your cardiovascular system whether bulking or cutting.

It's not "bad" and won't negatively affect your leg work. You can do cardio directly after a leg workout as long as your able to and not too sore from your workout. Alot of people tend to put it off for another day because of leg soreness while others use it as a way to alleviate soreness.

Keep lifting, eating right, and doing some light/moderate cardio to burn those extra calories and think you will be happy with your progress. Once you get to the point where you want to focus purely on either bulking or cutting then your outlook and training may have to change.