Technically you can't ready attacks outside of combat. I feel pedantic bringing this up, but there are situations where it matters.
Before going further, I just want to say that nothing needs to change for this combat because you got the drop on the Trogs and we can use your rolls as your first turn of combat attacks. In fact, having your attack rolls during initiative is a pretty slick way of doing it in PbP. However, I am uncomfortable using the term "ready action" outside of combat because it implies using your reaction outside of initiative order, which can cause problems.
For example, if anyone had failed their stealth rolls, then it should come down to who acts fastest to decide if the alarm goes off. If you can ready actions outside of combat then:
A. the players have readied actions to kill the watch trogs, and
B. the watch trogs have readied actions to hit the alarms
So who's readied action goes first? We should just roll initiative to decide. This is the classic "Han shot first" scene from Star Wars: A New Hope. Greedo failed his sleight of hand check to pull his blaster out, and it came down to an initiative roll, and nobody beats Han Solo in an initiative roll. Had Greedo been able to ready an action before combat began, he could have used his reaction before Han shot (even though Han won initiative) and killed him. Han stands no chance in a world where you can ready actions before combat, or EVERYBODY in the cantina would have to have actions readied. And then who's readied action goes first?
This is especially problematic in a turn-based group combat situation because you can alpha-strike so hard on a first turn. In short, readying actions outside of combat devalues initiative and makes things more complicated.
The way I prefer to do these things is to
1. determine surprise with appropriate ability checks compared to perception
2. roll initiative
3. take turns in initiative order, skipping creatures that are surprised on the first round
If you want to preroll your attack while rolling initiative because you know what you want to do already, that is fine. But it's not going to happen at reaction speed.
So, there's my pedantic rant ... in the end, it doesn't really matter for this combat :) But, hopefully I was able to describe why it could matter and it makes sense why I want to make that distinction in the future.