Is a piano a soloists orchestra?

the piano is in the string or percussion family

Let's just say it's on the list as an instrument available for use in the orchestra. However, the piano is not used on every single piece of written music for the orchestra! The piano will be used...

A percussive sound when striking a string with a felt covered hammer ... the vibrating string resonates a particular pitch.

Yes, but only when the score calls for it. The composer of the piece determines which instruments (and how many of each) are required for the playing of his/her piece.

A piano is technically a tuned percussion instrument, because its sound is made by hammers hitting strings.