As for BBM, version 5 will run just fine on OS6 - you just have to find the files to install whichever version you prefer.
Regarding hybrids, do make sure you're using the correct build over the recommended base OS. B@5tard is intended for an OS5 base, while my straight (and more prolific) B@stard series is intended to run over an OS6 base. I will admit that B@5tard is probably not my finest work - it had been a long time since I had built for OS5 and I don't think it is as fluid as the work I've done on OS6 builds.
My earlier builds handled the "page too large" issue quite well - look for my hybrids intended to run over 184.108.40.2065. I believe that is more a function of that base OS and less about my builds, but that may just be modesty as others' builds had those issues while mine did not.
In my experience, videos, twitter, and heavy browsing will take a toll on memory...but I have never seen a drop greater than 25 MB, even with uptimes of over 5 days. If I could settle on app versions (and be patient with my own building addiction) I would like to run a more thorough longevity test to see what sort of performance I see with even longer uptimes.
At this moment, I am at 3 days, 23 hours, 31 minutes uptime and my current memory "leak" is 24.5 MB (started at 97.8 MB free, now at 73.3 MB). My biggest memory-consuming apps are BBM, SocialScope, and Browser.