Siddurim usually print Psalms 78:38 and 20:10 (V'Hu Rachum) before Barchu at the beginning of the evening service. It is my understanding that it is desirable to begin a service with a psalm as introductory material as opposed to jumping right into tefilla. This is often given as the reason for P'sukei D'Zimra in the morning and Ashrei before the afternoon prayer. I think the ultimate source of this concept goes back to
Berakhot.32a Rabbi Simlai taught: One should always set forth praise of the Holy One, Blessed be He, and then pray for his own needs.
Berakhot.32b The early generations of pious men would wait one hour in order to achieve the solemn frame of mind appropriate for prayer.
With these things in mind, it seems different in the evening. I see people say the introductory psalm verses only when there is a minyan and they are saying Barchu. Otherwise, they start with the first bracha of Ma'ariv. This makes it seem like the psalms are bracketed with Barchu, not attached to the evening prayer like at the other two services.
Is the custom I have observed correct, and why are these introductory psalm verses treated differently? When praying at home, I have been adding these verses before beginning Ma'ariv. I cannot see a problem with this since it is almost never a mistake to say a psalm, but I wonder why this is not standard practice.