15 DC15255, Precision 5550, docking solution for multi-laptop swapping?

Unfortunately, you're going to be pretty limited with the DC15255s lack of thunderbolt. With the TB16, you're only going to be able to pass the mouse and keyboard as the USB C port on the DC15255 is only USB 3.2 Gen 1, meaning it does not support video output. So if your curved monitor supports HDMI, then you'll need to use that to connect to the Dell 15. This is a more cumbersome process requiring you to constantly unplug the dock from the 2 laptops. 

What you may really want to look into is actually a KVM switch. The 5550 should have Thunderbolt 3 USB C ports. I'd check out this switch here. Assuming the curved monitor has an HDMI port, then this should work out for you. Using a USB C to USB C cable (make sure its at least rated for TB3), you'll be able to connect the 5550 to the PC 1 C port. You'll then connect the HDMI cable from the Dell 15 to the HDMI on the PC 2 port. The HDMI on the back of the switch will then connect to your curved monitor and you'll be able to switch between the 2 systems at a push of a button. If the monitor only has DP ports, then you'll want to find a switch that does DP out instead. I think that'll be more tedious though since the switch will have to to convert the video signal from HDMI to DP which is active upscaling and typically is not the type of scaling you want to do.