3.5e has Swarms as a general subtype of monster, applicable to any large mass of Tiny or smaller creatures, but not as a template - there's no official method to do this. However, we can infer, from the examples given, guidelines on how to stat out a swarm. Based on the statistics of the base creature and the statted swarms given:
The swarm retains the size/type of the base creature (including AC/Hide bonuses due to size).
The swarm has some arbitrary number of hit dice chosen based on how tough you think it should be, I guess. A swarm of rats is given as 4d8, from the base creature's ¼d8; maybe a cat swarm should have 8d8, as the base cat has twice as many HD as the base rat. But bats also have ¼d8, and a bat swarm is only granted 3d8 HD... you have some leeway.
The swarm retains the speeds and methods of movement of the base creature. The listed rat swarm doesn't have the swim speed that the base rat does but that's probably an oversight.
The swarm's AC is as per the base creature.
The swarm's BAB is as per its new HD and type, though it cannot grapple.
The swarm replaces the normal attack of its constituent creatures with a swarm attack, described in the Swarm subtype, dealing damage based on hit dice; the hypothetical 8HD cat swarm would do 2d6 damage. If the base creature's attack has some special property, like Disease or Poison, this is retained by the swarm attack.
Swarms cannot make attacks of opportunity.
The swarm occupies a 10x10ft. space, with the caveat that it can take any contiguous shape that covers the same total area/volume - on the standard grid, meaning it can occupy any 4 contiguous 5ft squares. It has no reach (because Tiny creatures have no reach, and reach is meaningless for the swarm attack anyway).
Swarms gain the Distraction (Ex) special attack, where any creature that begins its turn engulfed by the swarm must make a DC 10+1/2HD+Con Mod. Fortitude saving throw or be nauseated for one round. Spellcasting, concentration, or using other skills requiring patience/concentration while engulfed by a swarm calls for a DC 20 concentration check.
Retain the special qualities of the base creature. As per the Swarm subtype swarm traits, it gains the special defensive quality of only taking half damage from slashing/piercing weapons if comprised of tiny creatures or immunity to weapon damage if comprised of fine or diminutive creatures, and all the other swarm traits described there, which grant it immunity to several effects and vulnerability to some others.
As appropriate for a creature with the swarm's HD, type, and ability scores/feats.
Retains the base creature's ability scores.
Retains the base creature's skills, including racial modifiers, feat bonuses, and size modifiers where appropriate.
Retain the base creature's feats, and potentially gains new feats as appropriate for increased Hit Dice (remember mindless creatures such as vermin never gain feats, but animals can).
Your guess is as good as anyone else's. Rather depends on how many HD you decided would be appropriate.
Alternatively, if your players really do get up in arms about extrapolating swarm characteristics from the given examples and statting your own, maybe you could just take the rat swarm and use the given rules for increasing monster HD to make it a bit tougher and call it a swarm of mangy cats, instead.