Flexible drops are very convenient for hooking up tanks. Obviously, not necessary, as the tanks are not moving relative to the headers, but convenient and insure no stress loads on the headers.
Use an isolation valve on both sides of the tank. At some point, you will need to isolate a tank--especially with new tanks as there may be leaks or other initial start-up problems.
At all costs, avoid using female PVC pipe-thread fittings over male pipe threads. At the very least, they will develop leaks, at worst, they'll split. Our local plumbing suppliers no longer sell PVC female NPT fittings in sizes greater than 3/4" for this reason.
We have sch 80 PVC glycol loops, some of which have been in continuous use for nearly 20 years. If I were building new, I'd probably go with the Cool-fit or similar, as a good insulation job is time consuming, difficult, and a gluey mess!
Insulation must be as nearly air-tight as humanly possible. Use a contact adhesive and tape that are approved for your insulation, and seal everything except for solenoid valves. As mentioned above, these tend to get tiny bits of stuff left over from assembling your headers and will need to be disassembled and cleaned, especially during the first year or so of operation. Locate your solenoids so they can be maintained in place, and skip the unions. Using the solenoid at the transition from the rigid header to the flexible drop, with an isolation ball valve between the solenoid and the header is the most convenient if you ever need to remove the solenoid. Unions will develop leaks after a few uses.
Remember that the solenoid always goes on the inlet, or cold, side of the tank. When closing isolation valves, always close the inlet side first, then the outlet side. Cooling jackets can be fragile and it's not a good idea to stress them unnecessarily.
Terminal Gravity Brewing,