I had a blast yesterday at our local OP6 event, and think I proved to myself that a Cardassian swarm can be "reasonably" competitive.
The "Kicking With the Cardassians" Swarm:
Koranak with Dukat 
Keldon class with generic captain 
Galor class with generic captain (24]
Hideki fighters 
I can't remember the exact build of my opponents, so I'll try to hit their main ships and upgrades
Game 1: Opponent was a good friend of mine playing a predominantly Fed build.
1) Enterprise D with Picard, Scott, Valtane
2) Excelsior with Sisko, Photons, Spock, Lojur
3) Hideki fighters
During the initial exchange Scott was used to disable shields for extra dice, and because I was able to concentrate on the Ent D with all four ships, I was able to eliminate it. The rest of the game was spent by both sides trying to chase down ships: his fighters trying to eliminate the Keldon, my fighters trying to eliminate his fighters, and the Koranak trying to eliminate the Excelsior. The game ended on time and me up on points.
Game 2: Opponent played a predominantly Klingon build.
1) Negh'Var with Dukat
2) Koraga with Picard, high sensor sweep
3) Hideki fighters
During the initial exchange I was again able to concentrate the fire from all four ships and take out the Negh'Var. Once the Negh-Var was out of the picture, I was able to reduce the fighters over a turn or two to where they were not going to be very effective, and finally was able to eliminate them and the Koraga. I can't remember if I lost a ship or not.
Game 3: Opponent played a two ship Klingon-Romulan build
1) Ch'tang as flagship with Picard, mines, and other stuff (about 75 points worth)
2) Science vessel with mines
During the initial exchange I was able to concentrate the fire from all four ships on the science vessel. However this didn't work, as even though I hit him close to 20 times over four shots, he rolled evades for every hit. I've been playing this game for about 6 months now, and I don't think I can remember anyone rolling more evades than what my opponent did in game three did yesterday. Between that, and me not avoiding the mines very well, he was able to finally halt the Cardassian swarm.
I'm mainly a pure Fed player, but for both OP5 and OP6 I wanted to change things up a little to see if larger builds with lower skill captains could be viable.
In one OP5 event, I actually used four Klingon ships with 1 rated captains and won one game. In the second OP5 event I upgraded two of the captains a little, but still played four ships and won a couple of games.
For the first OP6 event I went with the four ship Cardassian build, which I think is actually a little stronger than either of the four ship Klingon builds I used from OP5. In yesterday's games I never felt under-gunned or out-maneuvered. In game three I did get hammered by the antimatter mines and lost a ship to the station, but that wasn't the fault of the ships I was using. That was more pilot error.
I think the Cardassian swarm build can definitely be competitive.