Funnily enough, I found out pretty quickly just replacing the job class ACT file with the monster's wouldn't work because the action indexes are different (for example if you get hit, the death animation plays and you have to relog). What you can do though is use something like ACT editor and replace the actions manually with the correct sprites and that makes them fully functional. The only major issue is there's no way to hide the player heads or weapons without going through and making them all empty as was mentioned before.
I only recently learned about the disguise command too, it's certainly useful as well for checking sprites. Though in my case I was aiming for a complete sprite replacement of certain jobs which I've more or less managed so far aside from the aforementioned issue.
You may face some mistakes by switching a class's sprite to a monster's sprite.
In a class sprite there are positions that a monster does not have, sitting position for example, 2 or 3 types of attacks, etc ...
So, maybe you face some mistakes in that sense.
A monster has 4 layouts if I'm not mistaken, a class already has between 8 or 9 ... I don't remember for sure.
So it is necessary to pay attention to that
Someone correct me if I'm wrong, but I'm thinking about all the functions that a class sprite has
oh, there is the option for you to use the disguise command, isn't it interesting for you?
you can turn the player into the monster you want
perhaps it is a good option for your goal.
I think that's why it isn't working. The dev's team keep changing the code all the time and then the plugins become outdated.
I use an old revison of hercules, so that's why I am able to use all the plugins I want.