Ok! Here is my opinion for what it is worth. If you are just talking bluegills I would agree with a 3wt rod of what ever length would work in your environment, I would recommend a 8'-0" as a good all round 3 wt.
But, you said lightweight bass. I am not exactly sure what that means, but if it is bass that will weight less than 2 lbs. then the 3wt will also work for them. If you are talking about catching good bass that will exceed 16" and over 2 lbs. then I suggest a heavier rod.
I fished on July 26th in a farm pond in Pa. and I used a 9'-0" 6wt and had a ball catching blue gills and some bass that approached 16" in length.
Choose a fly rod for the size of the fly you expect to use and a rod heavy enough to land the fish quickly to assure its survival.
To catch large bass requires large flies and a large bass needs a rod with the backbone to land him quickly.
So, I suggest a 6wt as a do all bluegill/lightweight bass rod, and I can assure you the bluegills will still be a blast to catch on it, plus it will land your bass.
I fish smallmouths with 7 & 8 wts. due to the fact I throw large flies for 20" 4 lb. plus fish.
