By RAW there is nothing preventing a wizard of any type from selecting the Arcane Discovery Opposition Research. However, just because you can pick it, it doesn't mean it does anything for you. For example, a generalist wizard could take it, but since they don't have any opposition schools it doesn't do anything for them. In the same way a Thassilonian Specialist doesn't have any opposition schools. They have Prohibited Schools, which might be similar but they aren't the same thing.
That being said, I would work with your DM to see if instead of it making a single opposition school a normal school, it turns a prohibited school into an opposition school.
IMO it doesn't really fit with the flavor of the class.
Focusing their research on the discovery that each school of magic was opposed by two specific opposition schools, the runelords developed methods of further enhancing their mastery over their chosen arcane specialties. Essentially, they defined these seven schools as much by what they couldn’t do as by what they could. By excising “impurities” introduced by fixed oppositional schools, they traded arcane versatility for greater strength in their chosen fields.
But if it makes sense to you and your DM then that's all that really matters for purposes of the game you're playing.
When it comes to specializing in a subschool. If there is precedent of being able to do this (as with the aforementioned NPC), then it's probably fine. Examples IMO are strong evidence for what's allowed.
edit: Trying to get an official ruling on anything now that PF2 is out is probably about as effective as shouting into the wind. As piazo's efforts are much more focused on the new system. Your best chance at a semi-official answer is to ask James Jacobs in this mega thread. It should be noted that while he is the creative director for pathfinder, all he'll tell you is how he would handle it in his own personal game. Which may or may not align with the actual rules.