Questions Setting Up Sgt500 & Burkert Brass 200a

I would recommend this awesome article about corrosion and electrolysis
https://www.fastenal.com/content/feds/pdf/Article%20-%20Corrosion.pdf

As a general rule it is always better not to mix different metals to avoid corrosion.
On top of that, but I do not think it applies to you, is galling of Stainless steel, but that's when the thread is on the SS side. If you try to screw SS into brass I would suggest using teflon tape and teflon paste combined

 

1) For the application of CO2, the mixing of metals doesn't matter as much. It would be a different story if water was passing through, but hopefully, that's not happening in your CO2 system.
2) The amount of current going through the wires is minor ~167 mA, which is less than what goes through most phone chargers (those have thin wires also). Also, 85 PSI is way more than the max for this hobby (50 PSI max for those diffusers and much less for reactors).