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. I'm using sub directories as my choice for multisite in Wordpress instead of sub domains. Any ideas on how to do this?

EDIT: I know I could make the URL be something like http://ift.tt/1zHId3P but I'd ideally like to do it how I have it listed above. I also tried the plugin for the WP Multi Network and was able to acheive the nested structure that I would like with creating those subpages within my multisite defined network. However, when I went to http://ift.tt/1yFqS7m the page loaded fine but when I went to http://ift.tt/1D9wLin I got a 404 error.