Not all prestige classes are suitable for Entourage-Cousins. Many prestige classes rely on of being really good in PC class skills, or being 'chosen' by a deity - and those just don't fit with NPC class characters who have opted for a quiet(ish) life. However, there are a few, listed below, that can work well for SOME Entourage-Cousins. However, choices are limited, as not all the PRCs listed below are suitable for all Entourage-Cousins.
Levels are calculated for NPC classes. Cohorts (With PC classes) may well be able to take the class earlier.
A very rough calculation makes me think that en Entourage assistant will get somewhere between 500 and 1000 xp per game year - so if I take an average figure of 750xp per year, I would anticipate …
Advancement is slow, and it may be better to take a L6 PRC, rather than wait for a higher level class.