Sure, you can rotate all collections-- just as much as you can rotate anything else. Select them, specify a pivot, rotate. You can select all easily by right clicking on collection name in outliner (or you can just switch to that collection and select all, of course.)
Although, I would recommend creating an empty parent for the objects in your collections rather than rotating them en masse about some particular pivot. Just as a general rule.
The effect of moving to 8 bit per channel textures depends on what you’re doing. If you have a lot of bounces, it is possible for you to get some noticeable sampling error (which will show up as banding) from 8 bit color textures-- but for most use cases, it’s fine. (Think about, do you have a lot of transparency/refraction layers? A lot of volume layers? That’s going to be the big thing that makes 16 bit per channel better.) But for most cases, yeah, 8 bit is fine for diffuse.
Making your roughness/metalness maps 8 bit per channel isn’t going to hurt them one bit though. For that matter, although I know some people disagree with me, I don’t think there’s anything wrong with an 8 bit normal map.
Some of this stuff, you might be able to save some memory by combining it together into single images. Roughness maps, metalness maps are single channel-- so you can read them from a single RGBA texture using only two of those channels. For that matter, there are only really two relevant channels of a normal map; the third can be derived from the other two, assuming you’re working with normalized values. But that would mean nodes work, and I don’t know if it would actually save you memory or not.
File format can also affect texture ram, but I’m not sure if Blender actually supports any live-compressed formats (like .dds can have some texture compression that actually reduces the memory footprint and is decompressed in hardware from video ram, but Blender doesn’t support .dds, probably because it’s a Windows/DirectX thing.)