Can I use 38 special in a gun thats 38 super?

[Lycanthrope]

Yes, but the STI Edge is built as a LIMITED gun. It has no compensator and isn't set up for optics. It was built as a platform for .40 SW.

In IPSC or USPSA he can have a .38 Super, but it is automatically scored MINOR since it is smaller than the .40 needed to be scored Major power factor.

The .40 in the Edge is pretty soft. About like shooting a 9mm in a smaller polymer gun. Despite the Edge being lighter than a standard 1911, the weight is distributed better and that makes a difference in perceived recoil.

You can always tame it down further with a tungsten guide rod and/or barrel sleeve.