Well, TEC and Vasari have no logistic slots left (Advent have 1) and 1 tactical slot left (Advent have 2)...if you are really desperate for slots, I suppose you could "remove" the metal/crystal extractors and "replace" them with capturable extractors (like those in uncolonizable gravity wells)...that would give you 2 more structures slots for each faction...

Note that what makes a structure "tactical" or "logistic" is not where it is on the UI, so you could have 18 tactical or 18 logistic structures if you really wanted to...

Additionally, you could eliminate the culture centers and have all 6 factions spawn culture from their planets...

Of course, I kind of think there's enough structures already...