You can do an initial fill with tap water after you dechlorinate it. A lot of people who have large tanks will. It is the evap where you don't want to be using tap all the time. That being said, the water in Colorado is quite good and you could probably use tap and be just fine in most tanks, but why risk it? Our tap water in the midwest was like 450.
You need to discard the first 5-10 gallons of a new RO. If you are getting 150 TDS from the unit, then this is why.