Replacing construction grade 2x treads formerly covered with carpet, with oak tread. Finished skirt boards already in place. Underside inaccessible. Questions:

1 - Can either cut new stringers & install inside existing, or shim stringers at each tread 7/16" or so to adjust for thinner tread. Which way recommended? (first option might be default since removing glued treads might damage stringers.) Also considering scabbing on pieces of 2x12 with screws and glue to existing stringer as alternative than figuring out new stringer. Bad idea?

2 - Ideas for reliable tread template? For one job, Stair Wizard is overkill. I've seen Stair Template at Collins Tools. But maybe there's a slick way to do it without specialized tool?

3 - Planning on spiral finish nails and glue. Suggestions on glue?

I cannot figure out what you are asking? Why not try to rephrase your questions. #3 is simple, construction adhesive. But #1 is a ?

1 - treads are changing from 2x fir to 5/4 oak. Would it be better to shim each step of the old stringer, set a new stringer inside the old one or fasten new pieces of 2x12 on the inside of the old stringer at each step? Last option seems easiest -

2 - cutting tread to fit inside existing skirt board needs a template. Not sure how this is done without Stair Wizard or Collins Stair Template tools.

3 - what kind of construction adhesive - there's at least a couple dozen types?

IF I understand you correctly, you want to remove existing 2x fir treads and replace them with new 5/4" oak treads. So you want to have the new treads, which are thinner, remain at the same height as the old ones were, right? The answer is quite simple. The old treads were 2x stock, which typically is 1.5" thick. The new treads are 5/4 stock, which is typically 1&1/16" thick. To make the new treads sit were the old treads were, all you need to do is add furring strips of 7/16" to each bearing surface of the existing stringers, also called horses or carriages. This is the easiest and most secure way of doing it. Personally, I'd use 3/8" plywood strips; 1.5" x the length of the run, usually 10.5". Just glue them down and brad nail. Any wood glue will work fine

#2. I use a Stair Wizard jig. Woodwise make a cheaper jig.
http://www.woodwise.com/tools/jig.html

#3. PL 400 is the best. Liquid nails subfloor adhesive or heavy duty construction adhesive is fine. If you can get Bostick's Best in a gun tube, that would be great as well. Just make sure that the adhesive contains NO WATER. Haven't seen spiral finish nails. Trim head srews work GREAT but leave a hole about the size of a casing nail.

Hope this helps. BTW, I've built and rebuilt dozens and dozens of stairs. You can trust that doing it this way will work the best.

Thanks for the tips. Shimming would be the easiest for sure. I'm a bit concerned about the condition of the carriages after I pry off the old treads, which is why I thought up the other options.

If they are cracked or broken badly, then yes, you will need to repair or replace but I have never found that I needed to do that.