Do I want vegetable tanned and black oil based dye, or latigo or what?
Hmm, your 20-oz item, if it's what I think it is, should probably be secured against the back panel of the sporran so it won't be loose in there, and is easily reachable. So I would probably make the back panel out of veg-tan leather, probably 10-12 oz minimum. The gussets and front panel can be lighter and softer since they're just holding the other regular junk, and not the heavy item. For those, I would use an oil-tanned leather like "chap leather" or similar.

Latigo is way too stiff for sporran use, unless it's for the strap or some other structural element.