I want my corvettes to take the lead and my destroyers to stay back, but when I merge them together destroyers always stand on the front. Is there a way I can change that?
Merge your destroyers last. Order of ships in a formation is determined by the order in which they we're merged into the fleet, so you can have your Corvettes in the spearhead, cruisers in the middle and destroyers in the back by merging a pure cruiser fleet into a corvette fleet, then a pure destroyer fleet into that combined fleet.
I would not worry too much. Once you get into combat, the corvettes have a higher combat speed and will quickly move in front of all your other ships.