You don't need a fleet module to make a ship that steals aggro and absorbs practically everything thrown at it. You can have that and a wipe-this-map-clean space navy so long as you have access to HP enhancing modules, space-effective weapons and defences, and a serious chunk of cash to pay for the upkeep of said ships.
The way this works is, you add HP enhancement to the combat ships you want to move down the targeting list. The 'tank' ship should also have some HP enhancement since defences can't take care of all bleed-through damage, but it should definitely have a higher relative attack rating.
Although this means that your ships cost more overall, their survivability improves quite a bit. With two integrity grids a heavy fighter has 24HP base, then any bonuses (racial, trade goods etc). Even if a designated tank ship gets destroyed or somehow moves down the targeting list, your opponents may well have a hard time destroying the rest of the fleet, because you can outlast them in a fight. Unless of course they also have ships fitted with HP modules.
Fleet module allow you to do more or less the same but with a cheaper navy which can be wiped out easily without the tank.
