but where or when do the new monsters show up`?
The Skulltag monsters (dark imp, blood demon, cacolantern, etc) first start showing up in the "The Gantlet" remake (SSTMAP03). And they're in pretty much every map after that. Since they're more difficult, there are greater concentrations of them in the later maps.
and what does SST lite do exactly?
You can put it in your "Skins" folder and use it anywhere (even if you join a server that isn't hosting it) because it doesn't contain any protected/authenticated lumps like DECORATE or ACS.