I have a Thorax and I need a way to kill frigates with my corp. I'm the only heavy ship in the clan and I must defend myself as a leader. I have many players in frigates and really need answers so anything would help.
The best way to defend a cruiser against many frigates is to make heavy use of web and drones. Fit 5 Hobgoblin II's and one or even two T2 Stasis Webifier. If you need, drop the scram to have space for more web. Risky, but worth it. Also, turn off your MWD when shooting at the frigates. If you have a buffer tank, then it should hold a while on it's own.
If you are certain that there will be only frigates as enemies, I'd even suggest using a Vexor or an Assault Frigate like the Ishkur instead.
The Vexor can fit a lot more drones as backup ( in case your set gets destroyed ) and therefore is a better choice when going into pvp against a lot of frigate pilots.
The Ishkur can fit 5 Light Drones and has a ton of resistances as well as a lot of damage. Also, the light guns of the Ishkur are way better suited against Frigates than the medium guns of a cruiser. However, since it is a T2 Frigate, you might not have the skills for it.
Thoraxes are generally nearly as fast as some frigates, so use that to your advantage. Chances are you are using a blaster setup so basically your goal is to reduce angular velocity. Hopefully you can get a pretty good speed and optimally make them fall behind you causing them to have to give up angular velocity just to chase you, at that point you can pop them off. Usage of webs/scrams are highly recommended
Another idea is to spread the fleet out, putting each ship at around the optimal of each other ship. This means that most frigates will have to choose which to orbit, and the others will be able to pick them off because the increased distance means the velocity is less likely to be causing angular velocity against them.
If you are by yourself, and encountering several frigates I would recommend disengaging trying to jump through a gate while they are agressed and escaping. If they have you slowed, God have mercy on your soul.
Edit: After re-reading your question another thing stood out to me. The principle that you should be in a different ship size (larger) then your fleet mates is provably wrong. The best counter to a frigate is a destroyer, so I suppose if you want to lead them a Catalyst would be a better (and cheaper) option. However, I would advise you to build a fleet out of your corp mates that is mutually supportive. If you have more then a handful then you should have ewar, logistics, and combat ships involved.
If you are "leading" them (also know and fleet command, or simply "fc") I advise you to be in combat ship, preferably one of the same ones your members are in. When you are issuing orders it is extremely helpful to be in the situation to have a clear view of what the combat is like for those doing the shooting (ewar and logi are usually a bit far off).
Also, if you fly the most expensive ship, you are also making yourself a target, anything you can do to obfuscate your leadership role in the fleet to your enemy the better off you will all be.