Yes you can test addons on other servers. I've been using Vengeance for this. All you'll have to do afterwards is copy the configuration files, which are organized in the game directory by account/server/character. Assuming the account name is the same on each server, then you just need the character files at the end of the path. Move it to the end of the new path for Nostalrius. If the account name is different, then the entire path will be different and you'll have to copy a few additional files from inside the account folder. These files are inside the account folder, but not titled as a server.
For mods, I used this link to get a lot of them:
http://addons.us.to/addonsThere are other databases floating around, including a few Feenix addon bundles, but I don't recommend those, as many of the addons are actually outdated, meaning you could get better versions for 1.12. The site I linked here has the newest version of mods, but it's not a complete list. Some mods you will have to hunt for specifically