Its very poorly documented on a lot of the functions, yeah. Unless there is a big pile of docs hidden somewhere I haven't found. Forum search can help a bit for more info missing from the wiki, trial & error is a lot of it, and an outdated/somewhat buggy addon called zgoo to poke around the functions being used by the game and the variables associated.

At a glance, I don't think it would be possible to add numbering or icons for the response options in any clean way because of how they are delayed by Slow Dialogs (without changing the Slow Dialogs code to call UX when done with its part). The reason my addon works well with UX is mainly from the code in my own addon supporting UX (for example I specifically designed a function in my addon for other addons to use, which UX does), and its a lot harder to go at it from the other way.

If you want to make it work with slow dialogs for a temp fix by modifying your own copy of the addons, open SlowDialogs.lua and go to line 105 then add a new line "UX_Dialogue.chatterHandler()" just below it (without the ""), so it reads:
And then open UX_Dialogue.lua and remove the word local before UX_Dialogue = {} on line 5.

That should make the responses all have the proper icons/numbering and still show up delayed. It will not fix issues that come up pressing the repeat audio button, so I will see about a solution to that and put out an update.

That would be the only change needed to Slow Dialogs most likey, but it can be done with a little bit more code to check for if UX is loaded to let it work also when its not. I will send a pm to Shinni about including a check in Slow Dialogs if he/she wants.

Otherwise it would be a lot easier to just include "display as scrolling" option in my own. Its already using a timer and just not showing each word as its spoken, but instead waiting until the end, so it wouldn't be hard to have it just show as it goes. It would be a lot harder to try to track if Slow Dialogs has finished its work in order to replace the appropriate things from UX like line numbers/icons.

I actually asked Shinni if he/she was interested in collaborating to make them more compatible, and/or offered to make Slow Dialogs work with my own or integrate them together. That was in last April and I never heard back.

I'm not sure at the moment, but I will compare it in the near future and let you know. The main difference that I know is the compilation has a feature to hide the text on the stealth indicator. I kind of find the text reassuring, but I can see it being a little immersive breaking for others.

I'm not sure if the dialogue hiding part of dominoids has a timer like mine or is purely keypress as the original addon its based on seems to have been. I am currently considering ways to make my timer work better, but its adjustable in game with a slider and I've added keybinds for a toggle to always hide (voiced) dialogue text and one to immediately show text.

I'm going to be looking at the response options in UX Dialogue and seeing if there are new types that need their own icons now since it hasn't been updated since update 2 I believe and the API has had a lot of changes. I'd also like to make the title part adjustable in position (usually its the NPC's name) and the font for that and the main text able to be set (with different fonts from each other if wanted) from the settings.

My main focus for UX was updating it to work with the new API (without errors and warnings) and implementing the LibAddonMenu-2.0 settings page for it, and the rest of my time has been on improving my DelaySubs addon. I'm mostly trying to make the two highly compatible rather than merge them, so that acies's idea for how to repeat the audio and his nifty icons are still attributed to him and not mistaken for my own original ideas.

Oh nice, I didn't see that or I'd have probably just checked yours out. Since I already mostly sorted out the stand-alone and you're good with it, I'll leave it up and add a link on the front to yours for anyone wanting the compilation.