Would it be prudent to reuse the brine from a hot sauce for a new batch of hot sauce? I just made a really hot sauce and would like to carry over some of that heat to a more mild type of pepper fermentation. Is this just a bad idea?

Some people “backslop” a bit of old brine from one ferment to the next. It provides acid, some salt, and a head start of live lacto bacteria. I do it; probably up to 1/4 of the liquid in my kraut and hot sauces may be backslop. Never had a problem, and I’ve been very happy with everything I’ve made.

On the other hand, some people don’t do this. The type and concentrations of microbes changes through the course of fermentation, and the microbes that are alive in your old brine may not be beneficial to start a new batch. Possibly even detrimental.

As someone else said, give it a shot and see how it goes!

I don’t see why you couldn’t, but someone smarter than I would probably argue.

Why not split your next fermentation 50/50 with old and new brine. If it works, amazing. If it’s gross, well then You only lost half!

I recently did this (new batch was 100% composed of brine from an old batch).

I had big yeast problems that impacted a yeasty flavor on the final product so I wont try that again

you can, but you're adding late stage bacteria before the early stage ones have done their thing. If you want to use it to add heat just save it until you're blending the sauce and use it instead of the milder brine.

You definitely can, and should!

I just finished a batch where I did this (used 100% reused brine that had been in the fridge). I haven't tried any yet, will tomorrow!

The CO2-producing stage started on day 2, and the kahm yeast really got bad on day 5 or so, so I stopped then. For comparison, when using fresh brine I'd go for 7-9 days

I would mix em at the end. backsplashing increases the chances of molds etc. At least in my experience.