The unit declares its charge reaction as a unit (the entire unit stands and shoots or flees or holds). The unit also must declare its mutli-shot as a unit. I'd also say that this technique is probably not what GW intended in the writing of the rules, but if you decide to say that this is a bit 'cheesy' then stringing your skinks out in a line in front of your infantry should also fall into the same category, as they're not in the 'group' that the rules 'intend' for them to be. Use it if you can.
The only other option (though expensive points wise) is to stick a skink chief with Javelin in the unit and stand him at the back, as the whole unit has to be in range for shots to be fired, this should allow most of your skinks to be under half. Makes them a bit of a point sink for what they are, but if you had a huge unit of skirmishers....
I don't see how this helps the skinks at all. Characters and units shooting are worked out seperately aren't they? The skinks would open up with their blowpipes at long range, when the chargers came into range. As soon as they got into the chief's range with his javalin then he would fire. This would make the character and unit fire at the same time for purposes of the game. If you were using this argument with a champion I think it would be a more believeable statement. But since, unit champions must be armed with what the unit is armed with (at least for skinks), the point is moot.
I'm pretty sure that the charger moves until all are within range, so it does work. They won't stop and start movement randomly.
i just realized that the only time it wouldn't work is if the character has a weapon with a longer range because then he comes into range earlier then the unit
The tactic is a little cheese, but you see it on occasion with dwarves using heroes in handgunners with a pistol or (old ones forbid!) having a unit champion with pistols. The biggest problem I see is your dumping a whole lot of points into what is (generally speaking) a throw away unit, though I suppose you could have a unit of say 20 skirmishers... 40 shots (+ a Javelin!) would make a dent in a lot of things, though the unit would be getting close to 200 pts (including a fairly bare bones chief) a fair few points to be gambling on 6's