If it measures 1" ID, that is 1-1/4" Rehau not 1". 550' round trip is 24ft of head from friction loss alone, which puts you right at 8GPM on high with a 26-99. Figure in a handful of elbows and exchanger, you're closer to 30ft of head and out of the usable curve of a 26-99, meaning 4-5 GPM. Speed 3 is going to be your only option with that pump. 7,000 sq ft if all calling at once, is likely going to flatten that in a hurry and leave you with 50-60 degree delta T. Try it on high and check delta with everything calling simultaneously; if it stays above 140 on the return at the bottom of a cycle (say your setpoints are 170-180, need to figure from the bottom AKA 170) You'll be okay, but my prediction says you'll be looking for a bigger pump. Maybe something like a B&G PL-45