Regarding the pressure bypass and the temperature gauges, I'd caution from going too cheap- I understand that cost is critical for a start up- but keep in mind the price of glycol, the price of losing product, and the the price of causing damage to your chiller system due to losing glycol.
There's several pressure bypass valve options out there- The Kunkel series 19 or the Aquatrol series 69 are both models we've used and seen customers use and available in WW Grainger catalog. You're going to pay a lot more than $50, but you'll be using a valve rated for the temperature and pressure range you are needing. Looks like the tractor supply is a plastic valve that doesn't list any temperature ratings- this would make me nervous.
If you just don't have the funds to install the right pressure bypass valve- install a gate valve as a bypass between your Supply and Return headers and simply open this slightly to protect from deadheading the line and also protect from over pressuring the tank jackets. You are going to give up some efficiency by bypassing during times you don't need to- but at least your'll be protecting your equipment.
WW Grainger can also provide some good Thermometer options to consider.
Not real clear on your question regarding the piping- I'd run a common SUPPLY Header Manifold to feed all the inlets and a common RETURN Header Manifold that all of the returns will connect into. If your total Header run is only 25', the First IN/Last OUT strategy isn't too critical.
Good luck as you get your friends brewery piped in and off the ground. Hope this helps a bit.
Pro Chiller Systems
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