The Gorn are not agressive, canonically? Really?
In their only on-screen appearance they massacred every last inhabitant - man, woman and child - of a colony inhabited largely with civilians, knowing that they were civilians (or having the means to easily find this out). This is not agressive? It's a blatant war crime!
The word "civilian" was not spoken at any point in the episode "
Arena." Cestus III is described as an "Earth observation outpost" and its commander was a Starfleet commodore. True, they had women and children present, but the impression was that the Cestus outpost was like a frontier fort in the Old West (indeed, it was shot at a location representing a fortress, often featured in Westerns). A fort is a military outpost, even if it has a certain number of civilians within it. Travers' line "We had women and children" suggests a mostly Starfleet or government outpost with some civilian presence, not a colony that was primarily civilian. The mostly civilian Cestus III is something that comes along in the 24th century, once the frontier has become more settled.
The point is that the Gorn were not the aggressors, i.e. not the ones who initiated the conflict. The aggressive party in a conflict is the one that starts the fight, the one that invades or attacks. The Gorn believed they were being invaded, and they acted in what they thought was self-defense. Certainly they were ruthless in the defense of their territory, certainly they overreacted, but that's probably a species trait, intense territoriality. That's not the same thing as political aggression, i.e. the desire to attack or conquer others' territories.
My point is that, while the Gorn are certainly dangerous and ruthless when provoked, they were not portrayed as a power that has an interest in conquest or invasion. The point of "Arena" was that, though it initially looked as though the Gorn were attacking the Federation, that turned out to be a misunderstanding because they thought they were the ones being attacked and invaded. They were acting as defenders, not aggressors. Their perception of combat ethics clearly differs from ours, but it would hardly be the first time that the defending side in a conflict committed an atrocity.
And once they understood that the Federation was not their enemy, there was no further conflict, because the Gorn Hegemony's policies as a state are not aggressive, not defined by the pursuit of conquest. That's my point. After that initial misunderstanding, the Gorn have never been a threat to the Federation or to galactic peace, except for the brief period in
The Gorn Crisis where a militant sect briefly took power after slaughtering the legitimate leadership. Historically, aside from two aberrations, the Hegemony and the Federation have had either neutral or amiable relations. And the fact that the Gorn eventually ceded Cestus III even though it was rightfully theirs to begin with should prove that they aren't inimical to the UFP as a rule.
So there's no reason to assume that the Hegemony as a member of the Typhon Pact would be a potential threat to the Federation, because the Hegemony's leadership has never been shown to embrace expansionist or imperialist policies. They just want to be left alone, to protect their own territory. Yes, they're extremely aggressive in defense of their territory, but they're not political or military
aggressors, i.e. initiators of conflict or seekers of conquest.