So basically the Mods I have running work perfectly fine as is. However, if I add any new mod (no matter what mod it is) or even remove ANY mod (even one that doesnt affect any other mod) suddenly I get script errors with my existing mods.

The only thing I can think of is that the load order might be affected, but I dont know why this would cause such a ridiculous amount of errors.

To give a little more detail, it generally seems to be the Uviriths Legacy scripts that get all screwed up if I so much as add or remove a mannequin mod or anything whatsoever. This wasnt the case earlier, just something that started recently.

with such a problem it is always good to post the load order and the error messages

are you using any third party applications such as wrye mash or MGE?
also, try posting your warning.txt
we could probably help a lot more from that.

Are you using NoM and the NoM addon/patch for UL?

Or is it RoHT with a UL-patch?

Adding and removing mods from a MW load order is a lot like playing Jenga with lit dynamite. However, it can be done safely. First of all--are you using Wrye Mash? If not, you really, really should be, especially if you're adding and removing mods in an active save. A good rule of thumb when removing a mod is to always sync a save to your load list and do a "repair all" to remove references, scripts, and other debris the mod left behind. When adding a mod, the best place to put it is at the end of your load order, or as near to the end as possible; you can do this by using Wrye Mash to change the .esp's timestamp. Constant moving around in your load order can disrupt scripts and cause doubling and other headaches with some mods. (Especially ones with heavy scripting, such as a companion mod.)

If you're having trouble with a specific mod, it could be that it's particularly sensitive to changes in the load order, in which case see the above paragraph. Alternatively, you may have accidentally de-activated another mod or patch UL needs, or moved it to the wrong place in your load order. http://code.google.com/p/mlox/wiki/Mlox might also be useful to you.

Everything within a mod is given a reference number based on its location in the load order upon starting morrowind, when you save a game, every altered reference(this basically means, any items you moved, npc's you converced with, scripts that ran, ect.) is saved(which is why later saves are much larger than saves made early in game).

The problem is, that if you change your load order, and start up morrowind again, now the reference numbers applied by starting the game won't match up with those in your save, which causes the errors, you may notice other issues as well, such as doubles of npc's, spell casting issues, ect.

One of the best things you can do is install the MCP, and make sure to apply the save game patch, which effectively cleans saves every time they are made, however, this won't solve load order script issues, the only positive solution for that, is to not change your load order for any given game, and if you want to try a new mod, to start a new game to do so.

Do this.
And then update/repair your save with Wrye Mash after each change to your mod list, and basically all your problems will be solved.

Whew thank you guys! I really appreciate the help.

Yeah unfortunately im knee deep in this playthrough and the thought of starting over again because I didnt know UL didnt come with its own mannequins was giving me the shakes. Will do the suggested above, last question: what does MCP stand for nvm: a google search was all I needed