It is often referred to here that a LMB will eat BG 1/3 its length. So if you have 14" LMB, you should stock 5" or longer BG. However, I would harvest LMB first, then stock BG if necessary. Otherwise a lot of what you stock might just be expensive LMB food. Heck, harvest enough LMB and you may not have to stock any BG.
Warning: you are talking about changing the dynamics of the pond. Better check with the owners to make sure that really is what's wanted.
