Regarding the definition of intelligence, that is very much open to debate. Someone noted that they were looking for aliens in "whales, dolphins, and octopi!". I personally am betting on the octopi. Binocular vision, large brain, very weird looking octagonal symmetry. What kind of animal is that? Must be invasive, from somewhere else entirely. Even an alien would think it was alien!I guess, correct me if I am wrong, we still don't know whether we should call ourselves the most intelligent species on Earth. In my opinion, intelligence is relative.
Also, we have no physical strength, only intelligence.
And, that's going to be a problem if we try to be diplomatic to them. They should be much more fierce than us and will frequently fight. And, if my assumption is true, then, we should not think that they will be very polite to us.
Intelligence is certainly relative. But it should be defined by what the animal can do beyond what evolutionary pressures forced upon its morphology and behavior. Using tools is very unique in the animal kingdom, and we are many orders of magnitude better at that than all other animals on this planet combined (or so it would seem).
You note that humans are weak, which is true regarding earth's most successful predators. It is that large brain that makes up for it, of course. With all those tools. we can drop a bull elephant in full charge, or take out a sperm whale with little difficulty, regardless of what Melville might have told us.
Diplomacy is such a problem just among humans. So you brought up a very good point about using it with aliens. If they are here for hostile reasons, they won't be joining the UN, or any other deliberative body. Indeed, they will likely demolish it for no other reason than to demonstrate their frame of mind.
Now we just have to wait for some to show up and see what happens!