The Harp in orchestral music won´t be noticed that much if it isn´t either playing solo or with other instruments. It will definately not be heard if the strings and brass are fully playing above mf. But in those cases the Harp can be great as an accompanyment, adding to the tone colour. Btw, if you want the harp to be noticed as the main instrument, don't use the celesta. If you just want it to add something to the piece, use whatever you like
I actually have a great example, my orchestra is currently playing the Polovetsian Dances 8 and 17 (A. Borodin). And in the first bit of 17 (the softer, slower part with the very well-known melody) the Harp first serves as a rythm, playing only a's on the beat. After that the harp is doing arpeggio's on the beat with some solo woodwinds and some strings play between p and mf. In this part the Harp is very well heard. In the bit after that the Harp plays chords just after the beat, but the orchestra is playing around mf in which it kind of drowns, but still adds to the colour very well.
As a general advice: If you want the harp to be heard, use the higher register and just keep the other instruments soft and not too many.