Wth does the weight have to do with electronics?
These ESCs are made for RC planes, and they normally accelerate to top speed in just a second. When you have it trying to accelerate a heavy load, you will experience heavy voltage ripple and voltage spikes (they are not designed for that). You can add hall sensors and run a 6-FET ebike controller to completely avoid that issue, and it will be cheaper, but...it will also be larger and heavier. Much larger.
I'm so sorry guys. I really suck at this. If it's good for 10s, and I'm only running it on 6s, am I good or what?
You're good.
Is it ok to wire 6s1p 5Ah 20c zippy and 6s1p 5Ah 20c zippy (made from two 3s1p in series) in parallel?
If there is any voltage difference, there will be a rapid "equalization" when you connect them. Charge them both to the exact same voltage, then make the final connection through a large resistor, like a 100W lightbulb. Once they are equalized, they will stay at the same voltage as long as they remain connected. They will charge and discharge together.
Is the board just for neatness or what?
Yeah, its just for someone who doesn't want to mess around with a soldering iron. Easy to make your own balance harness.