There's a lot of repertoire by C19th composers eg Diabelli, Kuffner. Look on imslp (dot) org.
The issue about guitar and piano is not really just that of volume, rather, the sounds are very similar but not necessarily compatible, so not always easy to mix. Combining very contrasting sound is probably usually easier.
To start experimenting with this area I would suggest try making some arrangements, eg of folk or other tunes, finding out kinds of mixtures of texture and sonority work. The point about arranging is you don't have to think about melody or (mostly) harmony because they are given (more or less). Also consider arranging things for other instruments or indeed solo piano ... or indeed something from the advanced guitar repertoire, with the same considerations.
Simon Ambridge Series 40 (2005)
Trevor Semple Series 88 (1992)
Louis Panormo (1838)
Alexander Batov Baroque Guitar (2013)
Simon Ambridge 'Hauser' (2018)