Hello
So I wondered if there's a way to make it so that games installed via Galaxy and locally can coexist (i-e, installing games BOTH via Galaxy and locally)
Or isn't this necessary and locally installed games are detected by Galaxy?

You can point the Galaxy client to look into a folder and it usually can add any GOG game folders under it, but there a few exceptions, like the original Mount & Blade and S.T.A.L.K.E.R. SoC, which apparently lack whatever data in their installation folders that the client needs to recognize them.