A: Yes. Create a separate partition (APFS for newer macOS, HFS+ for Yosemite) and install Yosemite on the second volume. Use the Startup Disk preference pane or hold Option during boot to switch.
Introduction: Why Yosemite Still Matters mac os x yosemite 10.10 5 download dmg
A: The DMG is likely corrupted or incomplete. Verify its checksum against a known good SHA-1 hash (search for OSX Yosemite 10.10.5 SHA-1 on trusted forums). Redownload if mismatched. A: Yes