When I did a server(1.7.10 it never became public(Couldn't find admins)) you had to put default: false under options for every group that isn't default.
You could make a map that is like lava run but instead of lava you are running from police in like a city and you have a certain amount of time to find a way out of the city. But the all of the main exits have police. So you have to find a different way out. Hope this gives you ideas.
It could be that your internet isn't to great and it can only render some skins depending on how many players there is. I use to have this problem and then my internet got upgraded.