I learned in the past that files in a folder (mapp/POI/..) should be of the same make (HERE or TT). I never broke that rule.
Another rule of thumb, esp for POI, is that bigger file means more information.

As I wrote, I do use TT maps exclusively (the reason for that is found in this thread : [Only registered and activated users can see links. ]) in combination with HERE POI's. The reason for that is the size of the POI files compared to the TT POI files. I learned that bigger file means more information. However a smaller file could be as useful as the bigger file when the information you need is in there. The rest is superfluous. And even with TT POI files in the past I never missed a POI. Since SD are cheap, file size doesn't matter that much, does it?

I think for building is the same rule (size means information) applicable. I am not sure because I disable building views (even I have them on sd ) since I feel it's more filling the screen than useful. Not that long ago I found out that in some cases I found too many buildings blocking the clear view on the route on scree. That is personal and you may feel otherwise, I just give my opinion and can understand that screen size and personal likings can be of influence.