GShad are IMO not a good option for a small pond. TShad or GShiners are much better. There are lots of threads here on that topic.

BG do fine in tannic water. There are many thousands of tannic ponds across the SE US that BG do fine in. We have several that have almost black water with oak and lowland tannic trees causing the soil and water problems. The normal alkalinity in those ponds is 4-5 which is very low (acid). The natural BG population has been good for the 30 years I have been around them. Tannic waters do have low productivity which makes plankton production (food chain) low and stunting more likely.