I have read that some painters recommend one coat of clear before decals, then cleared, but I have only used sloar's method.

And since you've used Rustoleum, I'd go with a Rustoleum clear. And be sure to follow the can's recommendation on curing times. I've ignored them once, and got a nice crinkle coat as a result

Your issue now is, you've passed the recoat window on your enamel base coat, so your clear is not going to adhere as well as it would have if you did this all at the same time.