As has been mentioned above, just because you
can do everything under the sun in an iRule doesn't necessarily mean you
should. As is mentioned in the Ten Steps to iRules Optimization
Click Here making use of the right tool for the job is important. If your deployment dictates that your LTM is the best place to do this, then it can certainly fill the role, but whether or not you "should" do this here is highly dependent on your environment and deployment.
Also, Michael is 100% accurate: there is no hard upper limit of iRules you may run on a given platform. It's all about resource utilization, so keep an eye on that and you'll be fine. You might check out the timing command -
Click Here to help with that.
Colin
Nimbostratus