Unless you can get a state of peace instead of war the eye candy allied ships will only complicate the training. If the idea is to make the destroyer come at you so you can fire down the throat shots he pretty much has to be hostile anyway.
To reduce his effectiveness you could right click on him in the mission editor, select properties, and change the crew rating from the default VETERAN to POOR. If you want to keep the US ships but not let them sink enemy ships you could change their crew rating to poor as well, but again it's only gonna complicate things with random distractions. My Gone Asiatic career mod does that on purpose for the "chaos of battle" effect so you never know what's going to happen but for a training mission you need more predictability.
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