From: The Big Nowhere
In my experience, you don't really need a lot of Special Forces units in invasions. The problem I've found is that once I get armor units researched, the AI decides I don't need infantry anymore, so it quits making them. Also, once you build a robotic troop foundry, the AI will build only those units for some reason.
My solution is similar to what others have stated. Choose planets to home base my assault fleets at and order about 20 of each troop type manually at those planets periodically. It also seems to help if you build a starport at these colonies as the transports won't be flying all over the galaxy to refuel or repair constantly. I also mix my troop transport (in empire policies) to carry 40% Inf, 30% Armor, 20% Special Forces, and 10% Defense units and get a pretty good mix on each fleet.
Distant Worlds Fan
'When in doubt...attack!'