I go to gym once in 2-3 days, and I have a nice physique except my fatty stomach. Now I have started running daily for half an hour. But some of my friends have suggested to me that I should stop running, because it will also reduce your other muscles(e.g. chest and shoulder etc) with your stomach fat. I have really worked hard to gain some muscles and now I'm worrying does it really reduce our other muscles, as it reduces fat in stomach, by running. If that is the case, then what are other exercises that I can do to reduce my fatty stomach fast. Also I want to ask, whether walking at normal speed will help reduce fatty stomach.
Yes, combining strength workouts with non-strength workouts will reduce the effectiveness of the strength workouts. Of course, that should be acceptable if your goal is broader than pure strength.
Per Tom Kurz' Science of Sports Training, page 174:
Combining strength exercises and endurance exercises in one workout reduces strength gain without affecting gains in aerobic fitness... (Hickson 1980; Sale et al. 1990). Adding a relatively brief aerobic endurance exercise (a 3.2 k [2-mile] run) at the end of a strength workout lowers strength gains by 10% compared to doing strength exercises only (Hortobagyi et al. 1991).
However, Rippetoe and Kilgore note that there is a way around this issue:
...endurance work interferes with all the parameters [strength and power] athletes are concerned with developing. The most recent research into interference deals with the tendency of aerobic training to reduce the magnitude of anaerobic improvement when the two are done together or in close sequence.... Studies suggest that separating the two by as little as an hour blunts the negative effect of the endurance workout.
(Page 72, Practical Programming) (Note that strength and power, though related to muscle size, are not synonymous with that attribute.)
Just lifting will make you bigger and stronger than if you combine lifting with running, but running as well as lifting is a fine way to improve body composition (lose fat) at the same time as building muscle.
It won't directly, but it can indirectly.
You have to have a decent calorie surplus to grow muscle - otherwise your body can't build on what you've done in the gym, and you won't grow any muscles. Running alongside your workout can burn the extra calories that you were going to use to build muscle. This is probably what your friends are referring to. There's nothing inherent about running that damages muscle gains though.
Be aware: If you stop your cardio workout, you could hit a plateau very quickly. Your muscle gains are dependent on you exhausting your muscles. If you don't exhaust your muscles, your body will never get the signal that it needs to be stronger and bigger. If your heart and lungs are not up to the task of exhausting your muscles, you'll never grow that way, either!
There are two main approaches to balancing these demands:
Eat more. Calorie surpluses will mean growth. If you must do your cardio on the same day as you do your lifting, you will have to eat more to make sure you keep the surplus.
Alternate lifting and cardio days. Don't lift on days you run, don't run on days you lift (outside of warm-ups). This will keep your calorie requirements more stable, so the nutrition end will be easier.