To clarify a few things. Probes are not cloaked (says the guy the accused another of ignorance xD) But it is very useful to send out an early probe or SCV to scout.
You could say science vessels and ovies are the equivalent of observers, but science vessels are also important flying casters like the arbiter, so now they are arbiters too, and ovies (overlords) are also supply, so now they are the equivalent of supply depots too?
No unit is the direct equivalent of another but there are always units that fullfill jobs that needs to be done for that race (exept some cases like High Templar/Defiler, they do come in different tech tiers though)
I'm not going into the discussion since I already feel I ended it before. Just pointing out some technical stuff.