If you're adding a damsel to do your cycle, I'd advise against it. First, do a fishless cycle..... that way you're not putting a fish through the stress of cycling. Second, damsels are extremely aggressive fish & difficult to catch..... if you plan on adding additional fish later, there will likely be aggression issues, and you will have a heck of a time getting the damsel out without tearing the tank apart.
If you haven't put the powerheads in the tank yet, don't use them (return them)..... 2 x 265gph sounds like a lot, but it's really not. I had 2 Koralia 240's in my tank, and they hardly did anything. Wound up going with a pair of 425's, and later added another pair on top of that for my SPS.
As far as the refractometer, check online before heading to the LFS..... There's a few different brands on Amazon for around $30. I'm all for supporting the local brick & mortar stores, but for accessory items like this, they tend to really hike up the prices.