Yea, the normal turrets have a hard time with the exogalactic-strikeforce stuff. On the other hand, in my fs game when I was having a hard time with defending one flank I just plopped my _entire_ cap of laser II and laser III turrets (freshly unlocked because I wanted the better laser cannon modules for the spire stuff) down in one chunk covering that position, and they gave a very good account of themselves throughout the rest of the game.
But one of the keys to holding against the fs strikeforces is planting your cities on the wormholes that the AI have to enter to get to your homeworld; the city ffs block enemy movement and the city defensive modules in general are very strong (the weapon modules are notably stronger than the capital ship equivalents). Generally you'll need to back up the cities with some capital ships for sufficient firepower, and until you get to the advanced spire shield technology you'll probably want to layer some extra standard ff generators on top of the wormhole itself. The drawback of that is the 75% damage penalty for anything protected by a standard ff (spire shields, city or capital-ship, don't have that penalty), but in those defensive actions often it's more a matter of out-lasting the AI units than simply alpha-striking them to dust.
And, of course, it's vitally important to have engies under all the ffs (spire and otherwise) to repair any of them that get enough of a breather from the siege
