Yes, you can, however, not very many people would know about them. Most people tend to stick to the major addon sites that have tools that help addon authors. Sites like Curse and WoWInterface are better equipped to serve your addon to large audiences.
on addons ( steam/steamapps/garrysmod/garrysmod/addons)
Put addons in this folder: C:\Program Files\Steam\steamapps\<steam name>\garrysmod\garrysmod\addons
All addons for World of Warcraft can be found in the Interface\Addons folder inside the game's folder. If you put the game on an external harddrive, addons should be put in that same folder on the external drive. If you simply want to copy the addons over, simply copy the Interface\Addons and the WTF folders - they contain the addon modules and the settings, respectively. Make sure to change the name of the Account, Realm and Characters inside the folders if you are planning to use them for another account or realm.
Any addon is legal as long as it follows blizzards guidelines. Hence, most addons are legal.
You put it in garrysmod\garrysmod\addons
You have to find an addon first that is supported by cataclysm. Then once you have one you put it in the "Interface" folder in the world of warcraft folder. There will be a folder in Interface called "addons" thats where you put the addons. then before selecting your character when you are almost in game you need to click on manage addons in the bottom left corner and make sure they will be on. one good place to check out for addons is the curse addon site listed below.
You can get Addons from either: -Blockland Fourm's Add-On Page -Return To Blockland
How do you create torrents?
Yes, you can use any addons that a full account can use. There is no limit.
no
Make sure your directory looks like Steam/steamapps/username/garrysmod/garrysmod/addons If not, you may have installed the game incorrectly. Remember, the addons folder is for folders with an info.txt file. Other addons have folder names like "models" or "materials". Put these into their respective folders, like moving the contents of "models" into your existing "models" folder.
Yes