Well kae is often rated by the net to be up there for ret (on the
PP ret boards anyway, here is an example
http://privateerpressforums.com/showthread.php?146353-Best-and-Worst-Casters-in-Ret), not number 1, but up there. And if you collect ret, you'll probabaly end up with most of the casters eventually anyway, so it is no harm starting with that little lady.
eEiryss can be a great disrupter, especially in games at 15 points (where the enemy may have 1 jack, maybe 2), and retains more use against hordes than her prime version (atleast in my opinion). The Arcanist is always something that you can use, and allows Kae to get more shooting/upkeeps going by saving her 1
FOC (basically). And the models are really neat for the two of them, and even if they don't work out with your idea of a 15 pt game, they'll make it into your 25/35pt lists, so it'll be good to have them.
I reckon go for it, and good luck