for core choices in an all skink army it is a must to have skink cohorts with kroxigor in them, they are after all the best core close combat unit available to skinks armies. use their superior speed to out maneuver enemies to charge the flank, with the skink's poison you can weaken the front rank and have the kroxigors finish them off. words of the wise though, you want to get the charge so you can cause feasr, and use the skink's poison, and weaken the line first. if you do find yourself in the situation of being charged, you'll have fear on your side, but if that doesn't work always take the stand and shoot reaction even if you don't have poison, because you might get lucky, and even one casualty makes a difference. also, avoid big blocks of spears with your skink and kroxigor units since you'll have your work cut out for you having to take out two ranks, always strike first units too. this is where you skirmishers with blowpipes come in, try to stay out of the line of sight of enemy units near by and pepper the with poison darts slowly crippling them, and if your lucky send their big expensive units into panic. skink skirmishers should make up a large portion of your core. i find that skink skirmishers with javelins, and just plain skink cohorts, aren't very useful units, but that can be different for you, since it all comes down to your playing style.
with special and rare units terradons are great tactical units for march blocking and picking off lone characters and war machine crews, chameleons are great for putting on the scouting flank to take out mosters early on, or do what i like to call the toll both tactic where you put your chameleons in a hard to reach place, like a forest where they can bail quickly if things turn sour, near a high traffic are that many enemy units will pass and just pepper them with poison darts. kroxigor in my opinion are better as core units with skinks due to the fact that the skinks offer extra protection, and even when charging don't have to strike last because of the great weapon. i don't find the bolt thrower on the stegadon special choice very useful, but it makes up for that in its close combat abilities. there is a big debate between salamanders and razordons on which is better, in my opinion their pros and cons when put side by side say that in their own ways they are both equal, though i do prefer razordons because they look really cool. finally, ancient stegadons, as mounts for skink chief, and priests i find the highly effective.
as for units sizes for all these troops it is up to you, but i generally like to have many small flexible units.
camo-skink
ps. sorry for the text wall.