2 priests is two too many
imo. Keep one if you really have the need to use him as arcane vassel. Keep in mind that you can only do this with direct damage spells and Life lore isn't really the strongest lore for these type of spells.
Where you should put the priest? Either in a skink unit or on the ancient, in which case you need to upgrade to EOTG. A fun thing with a priest on a steggy is that you can challenge your opponents champs and watch as the stegadon stomps its victim to bits
I don't use skink priests at all when fielding a Slann. Usually the Slann is good all by himself and the arcane vassel, while a nice touch, is risky to pull off since it exposes your priest.
Overall, the lists looks ok, but I would strongly consider a banner or two on your Slann or/and the
TG-unit. Don't forget to calculate the
BSB onto your Slann, it's one of the major highlights with this guy!
Another fantastic addition could be a cowboy (either a scar-vet or a lord on a cold one, basically unkillable with the right items). This guy is your go-to-guy when it comes to warmachine-hunting.
Remember that winning is normally done by controlling the battlefield, and for that task Lizardmen have skinks. Skinks are godlike redirectors and can setup nice charges for your combat-blocks.
Happy hunting