Yeah, prefix only check the start of the ID, so Something.Umbraco should absolutely not be an issue as in your case. LP is by default too short I think. You can't reserve it, because it's shorter then 4 characters and I think they block it by default because of that. So they need to enable that you can use it. Or at least, that's what is looks like at the moment. The answer of NuGet support are...very confusing.