hi
you can connect many voltage-divider type sensors to the 5V supply, assuming that they do not draw too much current. There is about 400ma available form the 5V.
The thermistor setup you mention is a total of 12,500 ohms, so the current it draws would nominally be:
I= V/R = 5/12,500 = .4ma
So with 400 ma to go around, you could theoretically put about 1,000 of these thermistor circuits on the 5V supply form the Arudino.
The other question about the temp coefficient... they can be negative and positive, depends on the material used in the sensor. Also, if you put your fingers on the sensor contacts, then your skin resistance probably influenced the reading.
Check out this link: http://www.designinfo.com/cornerstone/ref/negtemp.html
D