1. There's three possibilities, both confirming Redjic is very old. Either:
A: He fought against the Saiyans when he was far younger and vastly less powerful, becoming aware of what kind of Ki signature they have.
B: He heard about them through word of mouth and possibly of their exploits in defeating Freeza, thus naturally assuming a humanoid as strong as Goku was a Saiyan.
C: He worked under Freeza at some point in the past when he was weaker, as we see a member of his race amongst the Freeza Force in the FnF manga.
2. Goku confirms he'd be able to get stronger through training and Redjic most likely expected a child like Goku to become far stronger when blossoming into adulthood. However, Redjic was already likely quite old and wouldn't have known about Goku's forms above SSJ. I have doubts that even if reaching his peak he could fight on par with M2 Goku's base form.