Driving the D5, gonna have some fun at your property!

Personally, I really like the forage pond idea. However, if moving over your own stock, you will need to be very careful NOT to include any GSF fry with your minnows/shiners. Unless carefully hand sorting, many of the little "fishes" look roughly the same at a glance. It might be safer to buy new forage stock if you have a trusted supplier. You wouldn't need to buy a lot if you just start broodstock in your forage pond and let them spawn to give you big numbers later.

The "paint tray design" is specifically to make it easier to walk and seine. If the kids did fall into the paint tray pond, they should easily be able to walk back up slope and get out. However, in Michigan you may need a "deep end" that is too deep for non-swimmers. A smaller pond will freeze much faster than a larger pond. If any fish are going to over-winter in your forage pond, then I don't know how deep it needs to be in Michigan to do that. I might want six feet?

Your forage pond could also serve as a "grow out" pond. For example, if you put in some adult YP and get egg production, you could remove the adults and raise a bunch of fingerlings with no predation. That might be great to stock into your main pond the next spring, but you would need the capability to overwinter.

Another option would be to push up your dirt high enough to build your forage pond such that it could directly drain into your main pond. That way you could just open a big gate valve and send fish over. However, that is going to be an ugly berm if your main pond is a visual jewel from your house or cabin. However, that would have to be a sealed pond (rather than groundwater). I can't remember, do you have clay?

As to the proper size, I think any supplemental forage would be a benefit to your main pond. If you have space and good material to work with, I would definitely err towards making it too large. Adding 50% more volume to a tiny pond is not that much more work with a D5. If it truly was too large, you could later divide it with either a net or a dirt berm.

I don't know anything about the proper technique to rotenone a pond. However, I believe in the threads that I have read, they usually do pull down the water level as much as possible?

I think esshup has managed or consulted on multiple fish kills. Hopefully he can give you some good advice.