Hope this is in right area, here it goes. I have model sprayed with Johnsson clear and waiting decals, after that they need to sealed in and get that flat surface back. So I have XF-86 flat clear and here's the question, can I thin it with Tamiya lacquer thinner or will this cause problems with Johnsson? Should I use X-20A acrylic thinner instead?

You should apply a gloss coat, to seal the decals. Once that has dried, you can apply clear flat. I would recommend X-20A, as a precaution.

That would probably be the safest bet.

So I decided to be brave and thinned XF-86 with Tamiya lacquer thinner, ratio 1:1 and it performed great. No problems with Johnson what so ever, and surface is flat as they come.