I haven't used Xizor in the latest meta, but I did well with him and 5 Z-95's before. I'm sure it's different with U-boats, though. Too bad they are the same PS 3 as the U-boats or you could go with a bumped up swarm. Even then...maybe it's worth it? They will still get to shoot and can help take down a U-boat. If you can get it so that all the U-boats can't focus on Xizor in the first turn, then you might be alright.
As for how to deal with Soontir Fel, I always gave Xizor the 2 pt Flechette Torpedo. Hitting him for the double stress will usually be enough to set things up for next turn.
