- Is it possible for a medium person to shoot (but not reload)
a small sized Double Crossbow with one hand?
- If yes, what would be the penalties for it?
More specifically, would you keep the inherent -4 penalty?
- Can said person treat a tiny sized Double Crossbow as a light
weapon for the purposes of Two-Weapon Fighting?
Asking for both your RAW and RAI interpretations.
Ranged weapons don't change categories of handedness based on size.
And honestly, those rules didn't really make sense when applied to melee weapons either (like a small spear still providing reach to a medium creature but becoming one handed).
My recommendation is to ask your GM how they want to handle it.
Personally, I would say no it doesn't become a one handed weapon, it certainly doesn't become light, and you definitely keep the penalty.
Using undersized weapons shouldn't avoid the penalties that the weapons are designed to have by lowering the damage dice, because damage dice are usually the least relevant part of damage once you get above level 6 or so.
- Is it possible for a medium person to shoot (but not reload)
a small sized Double Crossbow with one hand?- If yes, what would be the penalties for it?
More specifically, would you keep the inherent -4 penalty?
Nope. That's a property of certain crossbows, not of size. That said there's a clear correlation.
- Can said person treat a tiny sized Double Crossbow as a light weapon for the purposes of Two-Weapon Fighting?
Nah. Unless specifically stated that they count as light weapons, they don't.
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Asking for both your RAW and RAI interpretations.
I'd probably allow someone to shoot a smaller sized Double Crossbow with one hand, if they accepted the additional -2 penalty as described in the Light Crossbow description.
The RAI of the Double Crossbow was also that you'd never be able to full-attack with it (even with Crossbow Mastery), but I think we both know the way around that.