I can put a Pumpkin on most plants to protect them from damage. But Cob Cannons are too big for Pumpkins, and so the expensive Cob Cannons keep getting eaten by the Imps thrown by Gargauntuars. How can I protect my Cob Cannons?
It's difficult! Generally your best best is to plant something behind the Cob Cannons that will kill the Imps quickly, e.g. a Gattling Pea or Melon-pult. Unfortunately from recollection they will still manage to take one bite. Over a long game you will lose your Cob Cannon and have to start from scratch.
Your solution of Cattails is a good one to have in addition to the above. Actually if you are on a pool level you should plant your first Cob Cannons on the pool lanes, as those are not attacked by Gargantuars.
Insta-kills such as the Cherrybomb, Squash, Jalapeno or Cob Cannon itself work, though the first two require an open square next to the Imp which is unlikely. The Cob Cannon also takes a few second to fire so you would need to be very quick. And don't forget you can use insta-kills on the Gargantuars themselves before they throw the Imp in the first place!
I use three different general strategies, in varying combinations.
One is to keep squashes and jalapenos ready for gargantuans. The moment one appears, I drop a squash immediately on it/in front of it. Then I drop a jalapeno (preferably right behind it).
Another is to keep one or two extra Krnel-pults behind each Cob Cannon. This will sometimes give you that extra breathing room to take out the Gargantuan before the imp gets launched, or hold the imp in place long enough to kill it before it can chomp anything important. Two Kernel-pults will be much more effective than one, if you can afford the room in your strategy.
A final option is to create multiple Cob Cannons, and maintain one in reserve in case a Gargantuan appears. This requires some micro-management, since your ability to kill zombies may then become mostly reliant upon your Cob Cannons in general, but it can be quite effective if you can build them before you start seeing Gargantuans appear, or have sufficient defenses from one or both of the other strategies I listed to give you time to build up your Cob arsenal.
A strategy that I've seen is not putting cob cannons in the 3rd/4th column range and placing pumpkins with winter melons, gloom shrooms, and an umbrella leaf. The end result is a space where the gloom shrooms defend the cob cannon (the imps actually land on the spaces with the pumpkin so you don't even have to worry about them eating the cannon), the winter melons slow zombies ahead making it easier to target them with the cannons, and the umbrella leaf defends the unit from bungees. (This strat is used only when the primary attacking plants are cob cannons. If used properly could possibly get you to an indefinite flag, I personally have never used this but have seen vids use it properly).