To make sure that they get along, use skinks of a similar size and species. Enclosures can have several females, but males will tend to compete with one another. As a result it's best to give new skinks a trial run to see if they get along with the other skinks before moving them in permanently.
The difference is that a skylight filter is a basic colour correction filter with a slight magenta tint which has a similar effect on skies as a UV filter (which cuts out part of the spectrum), which is neutral coloured.
UV filters and skylight filters are often marketed as protection filters for your lens, but they aren't actually the same thing. In this video I explain the differences between the two and I'll help you decide which you should get: a UV or skylight filter.