I think it is more plausible the way it turned out. It is easier to become 'friendly' with someone you know better. The Klingons are much more above board - you know what you are getting with them so it is easier to adjust, even diplomatically, to be able to deal with them. With the Romulans, you never know what you are getting. The Romulans are the ultimate 'sneaky snakes'. It is harder to come to any common ground - and even if you do, you are always second guessing everything that happens. The Typhon Pact books are an excellent example of that.
I don't think that is true. That is what the Romulans became because of the TNG. The Klingons and Romulans switched in all respects. Even their ships the Romulans went from BoP's to big War birds akin to D-7 cruisers and the Klingons went from big cruisers (actual D-7 cruisers) to BoB's.
The Klingons were the sneaky ones. Yes the Romulans had cloak but they used ti to straight attack border stations, to be fair they were bad guys too. It just seems they had more a Roman ethos to them. They had the pursuit of honor, all the qualities that the TNG Klingons posses. Everything they did with the Klingons, for the most part would have been more realistic with the Romulans if we consider a Roman like culture. Its more odd with the Klingons considering how they are portrayed in the TOS.
In a way the TNG suffers because it does not have a viable bad guy they end up pumping the Romulans up, which if we really think about were not considered a MAJOR power in the TOS and creating the Borg.
It even makes sense in the TMP era, as Vulcan's are "brothers" to the Romulans and would give a reason for a coming together of the two groups. Consider if they had had the for thought of this. Spock could have had a relationship with the Romulan Commander from the Enterprise incident. Saavik could have been Spock's daughter, minus the pon farr scene in STIII. The Romulans are the bad guys in STIII like they were suppose to giving Kirk a reason to hate them. Spock's relationship with Romulans would now be very personal to the point that he would open a dialog with them in STVI and it would be the Klingons trying to prevent peace.