I wondering if you can rotate info decals, and if you can please tell me how. (I'm pretty new to mapping by the way)
You probably just have to turn off texture lock on the top toolbar. It should work after that. Edit: It seems that only works for overlays instead of decals, which are generally used more anyway.
Yes, decals cannot be rotated by design. They're designed to be cheaper to render than overlays, which is primarily achieved by sacrificing many of the functionality of overlays as well, such as scaling and rotating.
You can rotate decals using $basetexuretransform, not as convenient let's say overlays to manipulate but you might as well use overlays over decals when you're new to mapping.