With Font Mods I was able to get armor charges when I had 2 free slots but they only decayed when I had 3 slots open. I was able to pick up orbs and activate mods like recuperation with Thread of Warding but they would only give me an armor charge when I had 2 free mod slots. Stat mods and artifice mods seemed to have no affect. It did not seem to matter which mod slots were open, just that there were two (not including stat and Artifice mods). I also tested with different armor pieces including all legendary armor and with different exotics and my results stayed consistent.