Big load of questions there.

Focus firing with assailants is generally a good idea, especially with 10-20% phase missile penetration. When a phase missile penetrates it ignores shield mitigation, making them the best cap killers in the game. Combine this with the subvertors -10% mitigation AND 25% increase in phase missile penetration debuff and you can do obscene amounts of dps.

The special skill is a mixed bag. The AOE is nice (especially for killing masses of weaker ships) and the extra range is nice, but the autocast feature is HORRIBLE. If you do plan on researching this ability turn off the autocast and manually use it when your assailants are in position. Spreading assailants out makes it harder for opponents to fly through the grouping and shoot at their six, but spreading them out makes them even MORE vulnerable to the multiple firing arcs on illuminators and flak class frigates.

Generally assailants ARE the only ship a vassari makes in the early game. But the vassari player is always at a slight disadvantage fielding assailants vs. advent and tec once those players move on to their long range frigates. Because of this a vasari player, perhaps more than any other player, must retain an aggressive stance towards military research. When your opponent starts pumping out their own long range frigates you need to be in a position to bring out enforcers soon or be overwhelmed either by the efficiency of the LRM or the savage offensive power of the illum (especially if there is some malice thrown into the mix).

With that in mind the assailants serve their role as an early offensive unit. You can have assailants around very early on, making it very painless to clear out even the most fearsome of militia's. This means that you can generally beat your opponents to the punch and snatch a nice terran or desert world that by all rights should have been theirs. You can also mount a earlier offensive push into terran and advent players. When doing so you should try and take out any military labs or frigate factories. Hitting those structures can often times buy you the time you need to hit tier five and six of military research. It's at those trees that the vasari game comes together, the assailants just give your the harrasment force that you need to get there.