Hola Federico,
our CSS compresion tool is basic script, so it may not handle extra "rules".
For sure we cannot fix it on the forum, so I suggest using an external/extra installed plugin for compression and disabling JS and CSS compression from Helix Options. There is so many good working extension for that on Joomla Ext. Directory (JED), many also in Free version. You should be able to find alternatives pretty fast.
I guess, this time we cannot help more.
BTW
Maybe add those files using more classic way?
In Template Options > Custom Code > Before < / Head > and add < link HTML code to load those file(s)