The USA gets its sixth research slot from a path that requires either the "Limited Intervention" or "Neutrality Act" national focus. However, it's prevented from adding either the fascist demagogue or the communist revolutionary to their government if they have started either of those focuses (check the tooltips for those advisors), and both Limited Intervention and Neutrality Act will be locked/auto-cancel if you add either of these advisors.
But note: only Neutrality Act requires any degree of democracy support, so here's how* to get the 6th slot as one of the murderous tyrannical ideologies:
- Pick your preferred evil ideology and unlock its advisor through the focus tree.
- Appoint the advisor and leave them in place until you reach the necessary % of ideology that you want.
- Remove the advisor, then begin the Limited Intervention focus.
- Proceed down the tree to Scientist Haven. You must finish this focus before you actually...
- Switch ideology.
Seems like a lot of work for a single research slot, but you play the way you want to play.
*I haven't actually tested this because it's not my thing, but it works if the tooltips are accurate.