New Site URL for Multisite

I want to create new multisites but am having issues because Wordpress is saying it is invalid. I have a base url of something like http://website.com and then I'm trying to create a new multisite after that. I'd like my new multisites to have a name like:

http://ift.tt/1D9wNGX

http://ift.tt/1D9wLim

http://ift.tt/1D9wLin

Now I know these are typically categories in under the multisite but I'm using plugins that don't recognize categories unless they created for the specific plugin and I don't want to have to maintain the same categories all over so it's just easier to create my multisite with it's own database and not have to worry about it but Wordpress isn't allowing me to create URLs like above. Any ideas on how to do this?