Sounds like a great idea.
If they were activators then it's fairly straightforward.
Might also be possible to make a new baseform activator, then using FNVEdit change it to the baseID of what your cacti are currently using.
That way you wouldn't have to re-edit all cacti refs in the wasteland.
I don't think damaging creatures and npc's with them is such a good idea though.
The creatures are in their natural habitat... they wouldn't get to the size they are if they hadn't learned to avoid repeatedly headbutting the cacti.
The npc's can be dumb. I remember playing Oblivion and having a message pop up that some quest NPC was dead. I'd never even met him at that point.
Spoiler
There's an example of an activator that does damage on touch in New Vegas Steel. Those red hot bars of metal.
It always bothered me that Cass (I think it's Cass) says to keep to the roads as it's safer. Although that is where you bump into most of the trouble.
If the fauna were out to get you when you were unweary then yes... adds to the realism.