Adding BG to the mix would slow spawning, they will eat the eggs off the nest and you would likely get NO recruitment of SMB in the long run. A better bet would be to seine excess young of the year out periodically, concentrate on the 6-8" size, consider the smaller ones as food for the bigger SMB.
What time of year is the fish kill occuring? Winter time? How deep is the pond? This might help us understand the possible causes.
