Hey!
As Rosemow said, it's really up to you! Remember, this is your showcase, so you use it however you want. There's really no right or wrong! As for me, I have my thread very organized (because that's just how I am as a person) so if you look at my index for example I have it divided into 'Lots', 'Commercial lots' and 'rooms'. For lots I have put up everything I have created (old stuff too).Not sure why, I just wanted to include it all, since (as I also explain in the intro) it's all part of my journey in being a builder. For rooms I have only posted a few old rooms, the ones I liked the best. I have many more on the gallery I haven't bothered to showcase here.
So yes, I post it all in the one showcase!
As for Sims, I don't showcase any of them. I only have like 3 families on the gallery I think. I don't really make Sims (at least not yet) so I know very little about that whole process. @Corwim will definitely be better help on that issue!
:)