Using a MacBook Pro Charger on my Dell XPS 15 9550

Yes, since they both support the industry-wide USB Power Delivery standard, but you might find that the XPS 15 only draws 60W from the MBP charger even though the charger is capable of 87W.  Because of how USB Power Delivery works, the device drawing power will not always draw the maximum amount that the charger is capable of supplying.  Further, if you have that charger connected while the system is running, you will very likely notice performance throttling because the XPS 15 is designed for a 130W power source.  In extreme cases it may even continue draining the battery even though it's connected to the charger (just slower than it would have otherwise), but before resorting to that setup it will try to cut back on its power consumption overall by throttling performance.  So the MBP charger is perfectly reasonable to use if you want to charge the system while it's powered off (although again it will take longer than the 130W charger that comes with the system), but you may not like the way your system performs if you try to charge it from that charger while it's running.