The Seeker Scout is basically the same as any other scout frigate. You won't be using these for combat, so the hull and weapon strength of the this vessel isn't important. This isn't to say scouts are useless, because gathering intelligence on enemy locations is crucial. Just keep the Seeker out of combat, as it won't last long. With research, the Seeker can be upgraded with the Martyrdom special, which sends the ship on a suicide run. This is useless however, as the amount of damage done is not even enough to destroy an enemy scout.
The Disciple is the Advent's baseline light frigate. Compared to the frigates of other races, it is less expensive, weaker, more fragile, and quicker. Disciples are very easy to crank out in large numbers, but they will be decimated in any fight where they don't hold a large numerical advantage. This fragility, combined with their spend, make them best at hit-and-run tactics. Later in the game, the Disciple can be upgraded with two very useful abilities, called Steal Antimatter and Transfer Antimatter. Antimatter is what all units need to use special abilities. Using Steal Antimatter, Disciples can render enemy ships incapable of using their special abilities, and they will also be able to transfer antimatter to friendly cruisers and capital ships that have run dry. Using these two abilities, Disciples can be used like antimatter batteries, sap up the energy of the enemy, storing it, and then feeding it to your own units when needed.
The Illuminator is the Advent's light missile frigate. Except it doesn't shoot missiles. Instead, it uses long-range beam weapons that can focus on multiple targets at a time. They fulfill largely the same role as missile frigates, in that they are great at clearing out large numbers of weaker vessels from a relatively safe distance. In comparison to, say, a TEC Javelis however, Illuminators have shorter range, are more expensive, do more damage, and are much tougher. The Illuminator also has the Illusion special ability, which creates doubles of the Illuminator. They are fairly robust, so they're useful to have around. The only downside to the Illuminator is how quickly it becomes overshadowed by the Advent carrier, which is useful against a wider range of targets than the Illuminator.
Nothing fancy here - the Purge vessel is the Advent planetary bombardment frigate. Like most such frigates, it is weak against anything in the sky. It is quicker than other bombardment frigates, however. The Purge Vessel has no special abilities.
The Defense Vessel is really more specific than it sounds, as it is actually meant only to defend against fighters. Like the Purge Vessel, the Defense Vessel is nearly the same as its counterparts among the other races, and it has no special abilities. It very tough, and very effective against fighters.