Eddie,
I a little confused. Is this the screen you mean?

I'm not quite sure what instrument you mean by "solo instrument". If you mean the "Instrument" drop box where Tenor Sax is presently listed, these are only MIDI instruments for the MIDI solos.
When a MIDI solo is generated, it's a two step process.
Step 1 is to choose the type of solo and then generate.
Step 2 is to choose the instrument to play it.
Since Realtracks are unable to play single notes, this button is not relevant to them. Generating a RT solo is a one step process.
Hope this was what you meaning,
Noel