I'm planning to toss the fish that is below the legal size into my cooler.
Theres a reason that the fish is considered below legal size. Do you really think it's a good idea to keep it?
Anything to make the catch look bigger.
There's a reason laws like that are made, in this case it's to make sure the fish population doesn't get devastated by overfishing, which would lead you to have nowhere near fish. You should always follow the laws when doing something because they are set for a reason.
I'll return the fish.
That's good. Following the law is wise.