The Tyrant Ada fights on the highway is from Outbreak File #2's end scenario, "End of the Road." It's the T-0400TP, after it mutates. Before then, it looks like a standard T-103 (Mr. X type) Tyrant. This would place the T-0400TP fighting Ada right before it being absorbed by Nyx (another Tyrant from "End of the Road," who acts much like Uroboros and Birkin's final form). It's pretty weak when Nyx absorbs it.
Don't know about Sergei. He seems to be a unique mutation. Apparently, all those T-103s and the two Ivans are clones of him, since he has a compatible genetic structure to create Tyrants. (Or something? It's been a while since I played UC...) Wesker even muses in a file that the mindset of the host affects what happens to the mutation, so that could explain the difference between Sergei and the various Tyrants.