Just to clarify this a bit.. Decompiling a model is never a good idea because tons of things can go wrong, especially if you don't know what you're doing. This could lead to server crashes that are near impossible to debug.
The person that made the model would have the resources and expertise to make changes to the model without anything going wrong. If someone comes to me and wants a model modified in some way, I always point them to the original author for help because we simply don't provide any support for workshop models, we only add them to the server so you and your friends can use it.
The reason why we do not accept models that are not on the workshop is simply because these models tend to give us the most problems. A lot of times these models are from an older version of Garry's Mod, contain nude bodygroups, or is not allowed on the Steam Workshop for some other reason. Only accepting models and sweps from the Steam Workshop makes it easier for us to determine the reputation & quality of the content before we add it to the server. It might seem a bit lazy or careless for me to just say, "I will only accept models with a good reputation on the workshop", but you must consider the fact that there are hundreds of players on the server daily with new players dropping in every few minutes. It is my duty and a priority of mine to make sure content that is added to the server is safe and will not cause issues for our players. Server stability and performance is #1 over everything, including money. Unlike most servers, I do not profit on custom content. All of the money goes to pay server costs and does not go in my pocket, so I'm not really concerned with selling as many models as possible as quickly as I can. It is more important to me that I add quality content for players that will not break the server or cause players to crash. I can look at the workshop upload, read the comments, the rating, file size, etc. to determine if it's a good model to add.
To determine if a model is going to work best for you, you should test it out in single player first. How does it compare in size and height to the default citizen model? Once they are added to the server, models can automatically be restricted from raiding, revolting, or have reduced HP and damage if they are small or have wacky hitboxes.