The minimum army size you can take a Slann at is 1200 points., unless you play End Times hero rules (not many do) then you could take one at 600 points.
My general thoughts are that specialization is good, generalists are bad. Skink Cohorts (sans Kroxigor) are best for tarpits or redirectors. Kroxigor are for killing, especially monsters.
A tarpit doesn't need killing power. A redirector doesn't need killing power. A monster killing unit doesn't need units vulnerable to being thunder stomped.