Is this just because the time until the next block is mined increases with the difficulty level?

I was wondering if someone could elaborate on how increasing difficulty reduces payout? I'm having a hard time understanding that part.

Is it possible to mine both LTC and BTC in the same rig? I guess the root question would be is it possible to prevent bitcoin miners from using the graphics card and only utilizing the ASIC? Or would I have to buy a separate rig altogether?

Do you know where I can get asics for Scrypt mining? I was looking at the top performer on scryptpool for doge, and the person is getting about 250MH/s, and I'm kinda curious as to how they're doing that.

Hey, I was just wondering if these can only be used to mine BTC? Or can they be used for LTC and other various crypto-currencies?