So I've finished painting my Yamato, with tamiya paints ... And I don't know what thinner to use for the wash. Will mineral spirits ruin the tamiya paint job??
....any advice???

white spirits are the best if your gonna be using oils, they should not damage your paint finish in the slightest.
i use oils with white spirits directly onto tamiya paints regularly, and they never interfere with the base coats.

If you are using Tamiya acrylics then I would add a sealer coat of cler acrylic gloss (future) to help both seal the paint and give the Otis a nice clean surface to run over. I use odorless mineral spirits for my washes based upon te recommendation of smarter people here. Cheers.

Test on a scrap piece first, some people work well with oils over a gloss finish, others work well with oils on a flat finish. test both out and see what you prefer.

I always apply a few coats of Future/Pledge 1st, as decaling comes before any weathering. Then I seal the decals in Future/Pledge again. When dry, I start my weathering with oil based filters thinned with Mineral Spirits.
Joel