Ok, so I've been trying to use this http://rapidshare.com/files/437916985/Archangel.nif nif file instead of just a sniper with a suppressor but when I try to use this one instead of the original, the GECK crashes. Does anyone now how to fix this?

It isn't clear exactly which nif you are trying to substitute. If you substitute one nif with a different skeleton, the game won't be able to make any connections. Are you sure the old nif and the new nif are compatible?

No, I'm not, how do I check that?

EDIT:
I've also tried using http://rapidshare.com/files/437989561/sniperriflesuppressor01.nif which I made using a guide but instead of the mesh it just has a red exclamation mark, I realise this is probbably a different problem but I think I may have taken a step in the right direction?

The red exclamation mark means the game was not able to find the nif at all. Probably this means you have made a spelling error when entering the nif filename. Or possibly, the game was able to open the nif but one of its components, such as a texture, was missing.

For creatures and characters, there is an animation skeleton underneath, and you can only use nifs with compatible skeletons. I guess what you are discussing is a rifle, so skeletons may not matter. You may wish to contact the author of the nif you want, and ask them if some special steps are needed to use it.

Ok I've managed to get it in game (you were right it was missing a texture ) but it just appears as an ordinary sniper?

Edit:

I got it working! Turns out I just needed to change the world and first person models Thanks you've been really helpful!