A Star Silver from another airline isn't really "elite". United handles non-UA Star Silver members very similarly, by the way. Star Silvers board with zone 3, which is technically zone 4 because there is a "preboard" group that goes before they even call zone 1.

The Star Silver benefits don't stipulate priority boarding either.
http://www.staralliance.com/en/benef...silver-status/

The Security thing is hit or miss. The contract help that guards the "elite" security lines are kind of clueless usually. Just show them your boarding pass with "Star Silver"...if they even ask.