I like my 10 block
Here's the reasons.
1. Very Very few things are going to bean chameleon skinks very effectively with shooting attacks, and with their scout move into some godawful hole in the ground, they are unlikely to be dealing with main line troops. Being over 8 models gives you resistance to panic rolls, which despite cold blooded do fail.
2. Demolishing a target reactively. When dealing with something like fast cavalry or other skirmishers, it is critical to deal a strong blow and cripple the unit with a single volley, because with your short range you are likely to be shot at or more importantly, charged, on their turn. Ensure that they are so beaten up that they live in fear of your stand and shoot reaction. Having ten guys also means that with your fairly decent initiative, many things will also get punched before hitting you in addition to getting shot. That back line of guys swinging just helps you not lose.
3. More likely to cause panic checks on their own. Killing just that one extra guy can cause panic, since a whole lot of things you'll want to bean with skinks, like fast cavalry, hounds of all types etc. run in groups of 5-6 dealing two wounds is critical to panicking them. wiht 20 shots, about 3.3333 will be poison/hits when you move and shoot twice at long range since a lot of things have a 5+ armor for fast cav, that' 2.222. An only marginal buffer for your vital number. Any less and it would be less then average for you to panic such a unit! This will not do!