Welcome to PB and sorry for the late reply.

If the pond fills and you let it run its own course it should be just like what you have now only bigger. Most LMB/BG ponds in MS that are unmanaged or managed for big BG have lots of 12 inch LMB and some big BG/RES. The crappie are a wild card in your situation and could cause an unmanageable population. They could alternatively negatively affect the LMB population , the BG population , both or neither. It just depends on the situation and how it unfolds. Adding minnows will not help much as the LMB and crappie will eat them. You will end up with more small LMB and crappie with that approach. If you add enough to feed the LMB and crappie then you will have higher BG survival and thus less food for your big BG.

It would help to feed the BG. Move gradually to larger feed like AM 600 so most of the food goes to the big BG.

Good water quality always helps but be sure you need to fertilize first. If your alkalinity is below 20 ppm then fertilizer will not work and the excess nutrients can set the stage for serious problems later.

If you still have questions ask away.