What kind of boundaries do you think you'll need to bring in? Since you mention it in reference to the opportunity to spend more time together, it sounds like you may be using "boundaries" as code for "rules" and those can be a hindrance towards real growth.
I would urge against making rules to limit the amount of time you spend with them. Instead, focus on needs of each couple to have "couple time." i.e. you and your spouse without the other couple, and vica versa.
What should you discuss? Everything, as it comes up.
Frankly, dumb mistakes are going to happen. You are human, right? The important thing is to realize that everyone has the best intentions going in, no one wants to hurt anyone, and that dumb mistakes are not deliberate attempts to cause pain.
Gralson: my husband. Auto: my girlfriend.
Zoffee: Auto's husband. Cue: Zoffee's boyfriend. Bookie: Cue's wife.