50t / 38t Shimano Zee crankset double 10 speed casette old mountain bike commuter conversion I'm creating a commutter from an old mountain bike - a 2006 specialized rockhopper pro. 

I have a bunch of components hanging about and I have a Shimano Zee front crankset with a 38t and I have a 50t ring (104 bcd compatible). With long chainring bolts/nut I could get a 50t ring and the 38t on it. A set up a bit like a compact chainring on a road bike.

Rear is a 10 speed casette long cage slx. I have been running it wiht a 1x10 for mountain biking use but essentially I am trying to add a 50t on the front and still wrangle it so the chainline is ok. 

I also have a fairly old front mech bottom pull and a new-ish slx front shifter that is 2/3 speed compatible. 

Issues I may face: 
- distance between chainstay and 50t ring
- chainline 
- length of chain for 50t ring and biggest ring on 10 speed at the back.
- left shifter doesn't work with old front derailler.

Most mountain bike triple chainrings have a 48t large ring. Is there any reason I couldn't use the 50t ring?
Length of chain does not matter. The issue you will face is the rear derailleur's total capacity. Basically it's rear derailleur's ability to take up chain slack.

Total capacity = (difference in tooth count of front rings) + (difference in tooth count of largest and smallest cassette sprockets)

You can see the size of the biggest ring is not an issue, it's the difference between the two rings that counts. 50-38 38 is 12, which is less than standard road bike difference of of 16 (52-36, 50-34 etc.) so you should be fine with long cage derailleur.