My tiled shower needs to be repaired only where the walls meet the floor. While preparing for this by taking out the old grout, two small pieces of floor tile came out. Each piece that came loose is a corner piece (triangles - 2" x 2" by 2" - other tiles are 4" x 4"). Do I need to use thinset or can I use the same silicone I'll be using for the rest of the project?
If you are going to regrout the pieces that came off, you should use thinset.
If you are going to grout with silicone, you could affix the loose tiles with silicone. I don't like silicone for use in wet areas, once moisture gets behind it, the area never dries out. With grout/thinset, moisture can escape. Sealing grout is like using a moisture blocking/vapor permeable housewrap over your sheathing.