Harps, Piano Etc. In Orchestra

In an orchestra where many violins take center stage, wouldn't soft sounds like those of flute, harp, piano just disappear, or are they usually used for solo?

Not at all. Larger number of players doesn't necessarily mean one section is louder than another by default. Your question really deals with orchestration and how different instruments are used in conjunction with other instuments.

I would suggest looking into a couple orchestration books, The Study of Orchestration by Samuel Adler for example, to get an idea for how certain instruments are used in relation to others.

The quickest thing would be listen to some orchestral work along with the score and see how the flutes, for instance, are being used. A single flute can pack quite a punch due to it's sound quality/timbre.

Great place for public domain scores: http://imslp.org/wiki/

In an orchestra where many violins take center stage, wouldn't soft sounds like those of flute, harp, piano just disappear, or are they usually used for solo?

Of course they disappear, even violins may disappear when Brass enter, that's the point of learning orchestration and how to balance all those people there.

(Piano is much louder than Harp)