You don't need to kick his leg directly but instead just kick outside and past it as you bring your hip in lower than his pulling on his arm to bring him and his weight and balance up higher. If you are having to kick his leg and force this then that's a good indication that you are not getting into good position early on and low enough. That throw is nothing more than a hip throw with a sweep and I have always found it best to first teach students and fighters a basic hip throw and getting their opponent up on their hip in the "flying position".

I suggest you look at a basic hip throw (o goshi) and what I am saying here to help you better understand all this.

Depends on where you hit him. If his leg can't bend or move out of the way, then obviously something may break, either the contact surface or the target. And you're not necessarily kicking at a target, if you want to take their balance. You are going into a stance, and his leg just happens to be in the way and so your leg goes through his leg.

Nope no kicking with power necessary. If you need to kick like that you are doing it wrong and you could blow out a knee.

The leg movement should feel like a sweep like you are just brooming the leg away. To do this you need to get the weight off of the leg. The way I been taught I lift them on to myself during the entrance of the throw, so that large part of their weight is on me(like a hip throw) then you sweep the leg. Because there isn't weight on the legs it should sweep off without resistance. If you are hitting resistance that means you messed up on the entrance and you don't have enough of his weight on you.

Your not un balancing him enough and probably don't have him springing of your hip enough either.

if you doing a flying or wheeling harai goshi you likely to set your self up for a ride or blow out your training partners knee.

I was wondering should i kick his leg with whole my power , will it hurt him? I always try soft way but dont work. Is that allowed to kick as hard as i can? I am kicking with calf practicly so it should be good?