I have used minnows to fish in saltwater and used a minnow trap over night with dog food to catch them.  Can I do the same in freshwater?

Yes, but throwing a cast net is easier, less time consuming

and will result in large numbers of very fresh minnows.

8-)

I am looking to catch some big shad to stiper fish with where would a good spot to look for to throw the cast net?

If you have current, you will find them near rocks that create a break.

We catch ours along the dam wall when water is being released and

along the riprap in strong current. You have to find them shallow to

use a throw net successfully.

8-)

Yes, but throwing a cast net is easier, less time consuming

and will result in large numbers of very fresh minnows.

Check your local regulations before doing so!!!! I don't know what it is like where you are but a cast net is illegal to use in fresh water in California.