To answer a few questions, while I have read the entire Yuuzhan Vong saga (as tiring as it may have been at times), I envisioned the Xin as something far more alien and drew no inspiration from the books... same goes for Tyrannids, although I only heard of them, but don't know much about them. Of course, when you boil things down, all massive intergalactic evil invaders look alike.
There are differences, of course.
For example, the Xin are not from another galaxy - they are from the same galaxy as the vanilla factions. Sins take place in a relatively small cluster of stars, and while the Vasari may have had an empire spanning hundreds if not thousands of systems, that's still small fry compared to the size of an entire galaxy.
When it comes to Xin ships, all of them are alive, but not all are sentient. For example, smaller drones are capable of flying and shooting just like wasps are able to fly and sting, but they can't think for themselves. Larger ships, such as the Warmaster or the Chimera, are fully sentient.
Also, while the Xin ships are alive and some of them could actually fly and operate entirely on their own, most of them are controlled by the Xin inside. Drone ships, while analogue in size to frigates, are unmanned. Larger ships have Xin crews.
Xin ships are not built in space. They are gestated within water-filled caves within the mantles of planets the Xin colonize (even volcanic ones). When mature, they are transported directly into space via Apertures of different sizes, which are short-range wormhole generators (you can even see water spewing from the portal for a few moments before the ship appears). The larger ships are in fact filled with water, as the Xin are aqueous creatures.
Xin motivations are unknown. All that is known about them is that they annihilate entire worlds, have no mercy, are not very interested in diplomacy and basically the only way to stop them is to kill them. They are capable of using guile, but that is rarely required, since they usually can just overwhelm.
