Exposing animals to ammonia should be avoided whenever possible. I am a big advocate for dirt, but I would recommend against stocking your tank immediately after dirting it. Sometimes I don't get any ammonia spike at all, but it's not something I would risk - it really depends on brand and how long I've had the bag sitting around outside. I think your first idea is good: soaking the dirt and testing the runoff water for ammonia content.
I do not have personal experience keeping plecos in a dirted tank, however I have heard that they can distrub the cap if not given adequate hiding spaces. The cories are probably fine.
Good luck! I don't want to give you the impression that I'm discouraging you: dirt is the way to go, but getting the initial setup right is key.