If you're going wych cult then you need to look at ways of maximising combat success.
That means two things. Causing regular wounds (S3 wyches are going to struggle to cause wounds against anything T4 until you get furious charge or get lucky on your drug roll) and making sure that if you do win the combat, you wipe the unit wherever possible.
If i were writing a list therefore i'd do the following.
Armour of misery on the Succubus instead of the parasite's kiss. If you're winning a combat you really want your opponent to fail their morale check and the -2Ld it grants means if you win the combat, at best they're taking a check with a -3 modifier (not gonna help against marines and fearless, but anything else will work), particularly given the succubus ridiculously high initiative you're pretty much guaranteed to catch them and wipe them out.
I'd then be tempted to take phantasm and torment launchers (again, not gonna work on marines so you'd need to evaluate this based on who you're likely to be playing against) to swing combats your way before you even get there (any models not fighting back is a good thing).
I'd also drop the wych weapons, each one only affects one wych and whilst re-rolls are always good, they will frustrate you more than they'll help (shardnet and impaler wych will miss on 2's instead of ones, razorflails will fail to wound etc)
Bunch the reavers up a bit, maybe a squad of 6 with 2 blasters and a squad of 3 with a heat lance, put caltrops in every time though.
In terms of tactics, don't hedge your bets. Take advantage of cover yes, but the wyches need to be in combat as early as they can be, anywhere else and you'll be haemorraghing bodies really fast.