you should try to get them all to 2000 you get some nice items!
My progress report I just only finished the harpy quest most are level 10 or less centaur, Roper & Rafflesia are lvl 12, silver and brown nanabi vixens are both lvl 16 & Luka is lvl 11. so far I got every girl I can(I'm assuming I can't get harpy queen yet) and have ran across other then mimic, spider and candle.