Try not to get too hung up on which purpose each is for - there's a lot of overlap on kydex thicknesses. I think the manufacturers just make the kydex sheeting in different thicknesses and people use whichever they think is best.
The .06 is generally best for smaller and/or lighter things, whether we're talking about a knife, a gun, a cell phone, etc. The .08 is more sturdy, and therefore lends itself better to larger/heavier items.
I've made sheaths for large knives with the .06 before and not had any problems. The .08 "feels" better , but the design is the key. Many sheaths for small knives are made from thicker material, and as long as the design takes this into account, it works well. Some people like to use Kydex thicker than .08 for about anything. A blend, like TekSec mentioned, can really taylor the sheath or holster to the item, also.
Try some different thicknesses for different things and see what you like best. But if all you have on hand at the moment is one thickness, don't let that stop you from making a good sheath (within reason, of course!). :thumbup: