Most of the time, when LMB and SMB are both present (and assuming there is no supplemental stocking of either), the LMB will soon overtake the SMB. Several reasons come to mind:
1) LMB grow faster than SMB
2) LMB spawn much easier than SMB
3) LMB can eat more larger sizes of forage, and one main reason is mouth size
My limited opinion on Black Crappie is that if you want them, fine, but you have to understand them and have to actively manage towards that goal. Just throwing in some Black Crappie will mostly not result in a good BC fishery for several reasons.
Crappie are predators just like LMB and SMB. So assuming that you wanted a good Black Crappie fishery, you have to be able to effectively control the numbers of spawned Black Crappie, or be able to eliminate the "new" crappie.
One main way to do this would be to try an maintain some heavy base of other predator fish that would eat the young crappie.
But the Catch 22 is that those "other predator fish" would be competing for the same food as your target Black Crappie.
On a side note, I don't know if anyone has had success in pellet feed-training Black Crappie.
My comments are probably just the tip of the iceburg.
