It’s funny that you say that… just the other day I saw a report about plastics and PET bottles were actually a bit of a health concern. Tests show that bottled water in PET bottles contain a heck of a lot more plastic particles than found in the same water in glass bottles. This was not related to any specific brands, all brands suffer the same issue.
This means that fluids in PET bottles are contaminated by the bottles and while it is not yet known whether ingestion of those particles have a long term health effect, I really don’t think that trying to vaporize them and inhaling them could be any healthier.
Don’t get me wrong, I’m still using way too much PET and other plastics for my liquids, but I just want to point out that you shouldn’t just blindly trust your plastics.
The issue I have with stainless steel is that it can rust too easily, especially if it goes through heat cycles. Rust can easily flake off and be a contaminant. Otherwise, I don’t see any issue using food grade stainless steel for containers.
Why would you think that using a paint shaker wouldn’t do the same thing as any other method used to mix up your liquids?
If you’re going to let out all the air from your container and use the paint shaker, it won’t be shaken very well either, you need that pocket of air in the container for the liquids to be mixed properly.