[Basssiiie]

There probably is no limit on skins for SA-MP. The reason why this character (and others like Kendle, Salvatore etc..) aren't available as skin, is because they only have cutscene models, no ped models. This means that there are no walking animations, shooting animations etc. Their bones probably won't co-operate with the already existing walking animations either, cutscene models have much more detailed bones and animations.

Edit: Imagine Little Bill with the default CJ walking animation: this would mean that his legs and arms would be stretched out as if he was a man of normal height.