Do these games have missables?
Yes, a few items that only be grabbed when you visit an area for the first time and don't come back to later.
There is also a sub-quest line that is timed based your overall game clock for a character you recruit in the 2nd game.
There is also some characters that won't join your army unless you have certain party members on your team. The Final Blacksmith in Suikoden 1 for example cannot be recruited beyond a certain point.