I'm working fine tuning the existing planets' textures, as well as improving compatibility between Extrasolar and other mods.
If you want to use visual enhancement mods (scatterer and environmental visual enhancements) then Extrasolar will not work with Before Kerbin. However, if you are not using visual enhancement mods, then Extrasolar should work pretty well with Before Kerbin.
Yes, OPM and Extrasolar are compatible with each other. Although you can reach Valentine using stock parts, the journey will be much longer than if you were to use mods. Going to Valentine with stock parts is totally feasible, though.