A: Partially. Photoshop 7 will open PSDs created by modern versions, but it will ignore "Smart Objects" (they appear as empty raster layers) and "Adjustment Layers" created after CS6.
Keep it on a USB drive labeled "Emergency Design." You will inevitably run into a scenario where a modern computer restricts software installation, or your internet goes down, or your Creative Cloud subscription lapses. Adobe Photoshop 7 Portable
A: Most "full" portable packs include Adobe ImageReady 7, which is excellent for creating animated GIFs (a feature modern Photoshop has gutted). A: Partially