I hope you don't consider me going offtopic, but I have looked into simulataneous turn based games before - I made a 5-a-side football game (it was pretty shoddy), and was going to remake it, looking and playing a lot better.
Anyway...
Wouldn't you only have access to inventory for changing weapons etc once per turn
Well, if you consider what you can do in the inventory in XCOM, you'll have to support that. Which is reloading, throwing weapons, moving items, picking up off the floor, dropping on the floor. There's a lot of commands to clutter the UI
how far can you walk into the darkness ( UN-explored mapblock) in one turn?
What you'll find when you play FS is that you just set the destinations, and what happens is you unit will run at the speed (s)he's allowed to get as close to that point as they can in the turn time. However, when the turn ends, they retain the "plan" of getting to the destination, but they now start closer.
What this also means is that at the end of the turn, you can change the destination (if they didn't make it there), and it generates them a new route. You also allow them to plan multiple destinations (consider the waypoint for the blaster launcher), and per turn they'll slowly work their way through.
Amongst moving, however, in FS, is when an enemy unit is spotted, it automatically takes a shot (unless you set the player to "dash" mode, which ignores enemies). However, in XCOM, it'd be difficult to plan "reloading", unless you made it automatic if they were carrying ammo.
So there would definately be complications using Simul-TB with XCOM, some things you could work around (reloading), but some things I think would be too difficult (stunning and capturing aliens without firing at the first).
Grab FS, and give us a shout if you want an opponent. I really think you'll like it. I'm logged as "marqw", I'm not sure if server matters, if you want to add me as a friend in there