1. Yes, but you should get powered risers if you plan to have lots of cards.
2. It won't reduce any KH/s
3. There's no issue to begin with (see #2)
4. They are plugged via a normal power cable to your PSU (usually a molex connector, although there are sata ones too).
Thanks
but if (2) and (3) are not an issue, then why would I want to get powered risers?
Because you don't want all the extra power to go through your motherboard. It's the same reason anyone who uses multiple of any type of card (nVidia or AMD) uses powered risers.
Most motherboards are designed to handle maximum 2 additional cards boarded on them, which equates to additional 150w (+/-). More cards (75w+- per card) means more power is needed to power them - more current goes through the motherboard, which means increase in likelihood of frying said motherboard.