The easiest way to get addons and install them are with one of the two major addon clients, The Curse Client (requires a curse account) and WOWInterface's Minion.
To manually install an addon, you need to download it to your computer, unzip it if it is zipped and then move it to your %wowfolder/interface/addons/ directory.
You can get Addons from either: -Blockland Fourm's Add-On Page -Return To Blockland
They are addons for a specific class that you play. I play a priest so I get addons specifically for priests to make it easier for me to play my class.
Any addon is legal as long as it follows blizzards guidelines. Hence, most addons are legal.
you have to get the program wowMatrix from there you can get all the addons you need.
No, most addons will not get you banned. The only things that addons are allowed to do in game are approved by Blizzard. If Blizzard does not want an addon to do something, they just make it so that addon can not do it.
The number of addons you can have active at any time is limited only by how much memory you have (and the addons use) and if any of them conflict with each other. Otherwise, Blizzard has imposed no restrictions on the number of addons you can have in use at any time.
Mozilla Firefox have many addons, you can find alot of them on their site.
Addons can be found at the sites listed in the related links section below.
Depends on what kind of addons they are. You don't need Cataclysm for alot of them, though.
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.
Yes, you can use any addons that a full account can use. There is no limit.
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.