I use the same brutes and gunners mix that I use for raiding and it worked fine for the first 3 strongholds.
Well, if your ship holds 35 troops I might use 4 brutes, but if it only holds 20-25 you may want to try starting with 2 brutes and the rest gunners.
10 spaces would be hard to do, I suggest you don't do it and come back later when you can hold more troops.
Each Pirate Stronghold has a different strength and they seem underrated compared to players with the same strength ratings. You can go after the easiest one first (near Dolphin Island, I believe), then work up. They're a good source of gold and grog, so I'd recommend waiting until you can overwhelm them with 35 or 50 troops and get total victories before you attack them and eliminate them from the map.
My brutes are level 5, but at level 3 is when they get a total of 800 toughness and that really gives them the survivability they need to distract the defenses. My gunners are level 9.
Don't take on the stronghold with 1.6 million in Gold and Grog unless you have the storage space for the rewards otherwise you will lose the excess. One way to counter is to surrender when you have filled your storage so you don't waste them. I speak from the experience of collecting 1 million gold when I only had storage for 650K
