A way to handle this would involve making copies of the relevant floor tiles, changing those, and then getting this layout to use it. Which would probably mean splitting the floor into another submap (the same way e.g. hull is a separate submap) and getting each submap in inside/ and inside_damaged/ link also the proper floor variant. However, in my opinion it'd be better to simply alter the interior layout if you think this looks too bad (but I'm not stopping you if you'd be willing to do the work, I just don't find it worth the effort).
Hmmm ... I think that if you make this demo mod into a "full" mod, I'd like to include it with X:CE.
The worst technical problem I see is that the version in modinfo.xml shouldn't be so long, because it can't fit the UI .
But, given what I wrote above, let me list here some things I think would be good guidelines:
I think the larger a UFO is, the more it matters to have more variants of the interior. You did this the right way of starting small in order to try and learn, but small UFOs are simply too small for variants to matter much. On the other hand, fights e.g. in carriers take much longer and so there diversity matters. So while working your way up I recommend not spending too much time on the smaller UFOs.
Note that for larger UFOs the command room is marked specially (it's the "Command Room" button, works similarly like "Paint Tile" but is a toggle). You'll also need teleports but I hope you can figure them out easily (besides placing the teleport objects, the tiles also need to be linked using matching IDs, see the bottom buttons row in the submap editor).
You might find it useful to reuse objects from alienbase layouts. You cannot use objects from other tilesets directly, but you can copy them. So in order to use e.g. tiles/alienbase/props/UFOwalltube object, copy the _spectre.xml plus all the images it refers to inside to let's say tiles/UFO/copies/alienbase/props/ , and modify the _spectre.xml file to refer to the images using correct paths. That should be enough I think.
I'm not a fan of small rooms. The reason the Fire in the Hole mod is not included with X:CE is because I think it alters balance, but primarily because of its small rooms. That was after seeing a YouTube video where the player very easily cleaned up a whole UFO simply by breaching a room with a shotgunner, shooting the possible one alien that was inside before it could do anything, and did this room by room, while the defensive aliens inside the UFO didn't really stand a chance. The defensive alien AI is simply not good at making counter-attacks, which would be probably the only way. I'm not saying small rooms are wrong as such, it may be good to have some to have variety, but I don't think an amount anywhere near FitH would be good for the way the game is now.
Speaking of that, apparently the hardest UFO to breach is actually the corvette, because of its entrance. So I think it'd be good to have layout(s) for larger UFOs with similar ideas somewhere, not necessarily again as the entrance.
I like things to have their logic and consistency. There are limitations to this e.g. because of technical reasons, but there shouldn't be things which are obviously strange, such as a UFO's command room being on the ground floor, or a maze of rooms done just for the sake of it. With the alien base maps I created recently, the first step I did was always having an idea about what the layout was about and how it fit together. In your first layout, the added wall makes a lot of sense, it separates the technical part. But the second one makes me wonder if the aliens would really build a UFO like that, or if was human thinking that lead to the layout. I'm not saying the layout is wrong, but it makes sense to occassionally do a check, especially with larger maps, whether a map doesn't end up being artificial, "cool" but non-nonsensical, or something similar.
Each layout should get at least some practical testing. Besides the obvious reason of finding mapping mistakes, it'd be also good to check things like whether the AI doesn't end up doing something really stupid.
Thank you for this and I'm looking forward to seeing what you come up with. If you'll need help with something, just ask.