And again, when I mention the peace between Ferelden and Orlais, that was 9:30, just before the Blight. Point being that since the two countries were presumably at peace (Bryce and Eleanor Cousland had recently returned from a visit to Orlais) there was no need for a "peace agreement."
Peace was formalized in 9:13, I believe, when Celene became Empress. According to the wiki, at least. An official peace. So yes, like Monica says, Orlais and Ferelden were at peace. However, it wasn't a particularly stable peace. Gaspard and other Chevaliers and nobles did seem to want to conquer all of the territory Orlais had lost, including Ferelden.
Cailan's reference to making peace means sealing the deal rather than them both licking their wounds and preparing for the next invasion of Ferelden. As was common in medieval Europe, this alliance would be sealed by marriage. Cailan and Celene would produce a child that would be heir to both kingdoms, theoretically uniting them in interest, since both kingdoms would be the property of the other's royal family. Leon-Castile and Aragon pulled this off successfully, becoming Spain. However, a lot of these unions became quite messy, with one country rejecting the other, trying to become dominant, putting forward a powerful noble for the throne as a non-treacherous replacement. (Couslands would be perfect for that.)
For countries with as bitter a history as Ferelden and Orlais, and so recent, and two countries so different in culture and governing practices...
Cailan had an idealized vision of uniting the two countries and them both learning from each other. Celene wanted a secure border to the east, the reclaiming of an Orlesian territory, and more soldiers to ward off Tevinter and seize the territory Nevarra conquered during the Ferelden conflict.
As mentioned earlier, there is even precedent for Orlais sending soldiers into a country and using it as an excuse to annex the territory. Loghain was right to be paranoid about the Orlesian forces. While understandable, it was a much less astute call to outlaw the Grey Wardens and refuse them entry.