I had the same, 7 Harem Girls with Zara so i needed to dismiss Zara first to get access to all the others and the next paymend Zara showed up again in the list.
zapallbugs There is no cheat for this to happen, when Zara is in your Harem you are able to recruit 6 more Girls = 7
and yes the Game is not designed for more then 6 Girls, but this semes to be a little Bug in here, b.c. the oprion to recruit a Haremgirl is still acrive with allready 6 Girls. There should be a stop for this option untill you dismiss a Girl to the count of 5 Girls