The usb miner will draw the power it needs directly from the port in which it is plugged. They only draw a couple of watts, so there isn't too much of a concern in supplying enough power. However, be careful which usb miner you would buy as some require the additional power from usb 3.0. If you only have usb 2.0 available, you'll need to look for something that doesn't exceed 500mA. I haven't kept up with all the specs of the different usb miners, so you'll just have to verify the power consumption on the miner you're looking at. There is no loss from adding additional miners; so yes, if you had 2 that each did 2.7 Gh, then you'd have a total of 5.4 Gh.

Generally speaking, the standard unit of measure for BTC is currently in Gh/s, but that will be changing in the coming months to Th/s. For scrypt mining, such as LTC, the standard unit of measure is an order or two of magnitude back. One video card, for example, will mine in the Kh/s range while someone with even a couple GPUs will be bumped up to Mh/s. It all depends upon what currency you'd like to mine. Those usb miners will only work on sha256 protocols, but if you're calculating for hash speed in the kh/s range, you must be asking about mining with a GPU on scrypt...?