Stop me before I do something really stupid

Hi guys
just beginning my first fixed project, got an old mountain bike, with suitable dropouts. just like to run some things by you as i am not very familar with doing this kind of thing.
My bike has a 130mm spacing between the dropouts, was thinking of getting a disc brake hub with a bolt on cog from london fixie bike shop, this is mainly because seems easy to set up, and easy to swap cog sizes. if i buy one for a 135mm spacing will i have any problems, i presume just few mm of axle stick out to the side?

Contact/email london fixie bike.

i am worried about my chainline, read a few articles, sheldon etc... seems very important. if i run a single chainring cog, where will that sit compared to my three cogs at the moment, i presume same as the outer larger cog, which i measure chainline at about 55mm, seems big??? most websites etc talk about chainlines in the low 40s. but as long it matches the rear cog doesnt really matter, does it??

any help appreciated before i splash the cash on something really silly..

Seb

I am not sure any of your questions are formed clearly enough for me to offer any advice ! :( Sorry.