I contacted Bill D who has a 0.5 ac pond where he stocked SMB and LMB. Bill said he has not caught a smallie in two years and no evidence of meanmouth bass hybrids. I think the reproductive behavior of these two species is the main factors that keep them from creating hybrids. Meanmouths of SMB X LMB are apparently a very rare occurrence and with a very slim chance of it to happen in a small pond. However I never say never when dealing with fish and other pond critters.
A customer has a pond where we stocked 50 LMB in it and nobody was supposed to have fished in it for about 4 years. I saw 2 LMB follow a lure to the shore that were about 4# each. Then the owner decided to stock SMB in there. No visible signs of any hybridization, there are just SMB in there now.