The aliens in Ship of Tears are not Streibs. They are Zener. I believe that the Streibs were originally supposed to be a one-time only plot device kind of like the aliens from A View From the Gallery. Honestly, though, I'm a bit surprised that the only recurring Shadow allies were the Drakh and Zener (NOTE: The Zener are the aliens you see prepping humans to become CPUs for the Shadow vessels and also are seen aiding the Drakh in Movements of Fire and Shadow testing Londo to make sure he could be joined to a keeper).
The Wurt were mentioned in the Technomage trilogy, but I have never heard or seen a description of them. I may just consider retconning them into as the aliens from A View From the Gallery since their ships look to be possibly influenced both by Drakh and Shadow design philosophies (well the fighters anyways).
I'm pretty sure that the Streibs have been retconned at some point to be allies of the Shadows since they embrace the art of conquest, though, seriously, these guys seem pitifully weak. I would be surprised if these guys were even noticed by the Shadows.