Would it be feasible to use armor spacing as fuel storage?

That would depend on the pressure that needs to be maintained. But I would worry about 'water hammers'. Using your armor as propellant tanks is fine unless it impedes it's function as armor.

Bigbombr, I wrote a long reply to your post but for some reason when I posted it, it was just an empty box. I'm short on time so I'll be a bit briefer now.

I'm aware of the need for an oxidizer in combustion. But if your ship is hit and your armour breached, there is at least the possibility of damage to the inner hull resulting in an atmosphere leak.

I know that my description specifically applied to combustion rockets. I thought that was clearly implied by the reference to real-life spacecraft, ALL of which to date have used chemical combustion rockets. I didn't mention NTRs or MPDTs because I have only a pretty vague knowledge of how they work. Part of what drew me to this game was an interest in, and desire to know more about, real-life or realistic spaceflight. This seemed like a more enjoyable way of learning than relying on textbooks alone. But there are still gaps in my knowledge, and I try not to comment on things that I don't at least roughly understand.

I also know that separating fuel and reaction mass is bad in terms of spacecraft efficiency. BUT there is a case for it, perhaps if for some reason fuel is scarce or expensive, or for a ship that isn't intended to alter its trajectory much, and which has low enough mass to reasonably be delivered to its location by another ship. In that particular instance, lower thrust might be an acceptable trade-off for NOT having to carry a huge amount of potentially explosive fuel. The case for using water as propellant is much the same. It's cheap, easy to come by, and it's hard to envisage how carrying it can be dangerous. So if you REALLY don't need a lot of mobility, it might be viable. I do realize that in normal cases it's a bad example to give, but it was just the first one that came to mind.

To your last point, both chemical and nuclear fuel shouldn't really be bolted to the outside of your ship if you can help it, at least not if you're expecting to ever get shot at. I'm aware that nuclear fuel is probably less hazardous in this context, but you still don't want it floating away when somebody shoots a hole in your armour. In extremis, one could use water or even atmosphere from the life-support system as propellant. But if you lose your fuel supply, whatever type of engine it powers, you have a lot less options.

This makes sense. So you'd ideally use up this 'additional propellant' before encountering the enemy. It would essentially act as drop tanks you don't drop.

Keeping propellant in between armor layers during combat seems like a very bad idea.

Second, unless venting imposes too much thrust on the ship (and compartmentalization helps a lot here), penetrating armour tanks doesn't have severe implications.
Yes, you do lose delta-v, but if you're using ships that are built to get hit and survive, you're either trying to establish presence somewhere or have presence there already - no matter how much delta-v you expend, you end up with ships where you wanted them and presumably within reach of assistance - any day when you're still in orbit around the same body after the battle is a good day.
If you just want to hit something without establishing presence you don't send ships into combat - you send unmanned missile buses and drones, if there are any manned ships in your fleet they remain in distant fly-by trajectory doing their best to stay out of reach.

Nuclear fuel *tanks* are inherently worrying.

Although the game doesn't consider processing fees, I am pretty sure an armor like that is not going to be easy to build in the real world.
And, you are not going to find equations about liquid armor on internet...right?