Hello,
None of the above. Per p. 146, Jihad: Final Reckoning, The only acceptable combinations of multiple items per crit slot are heat sinks (of the same heat sink type) and ammunition (of the same weapon type).
And, yes, a critical hit affects all items in the slot (for multiple ammo slots and heat sinks, this means a critical hit to everything in that slot, be it two ammo bins, a single Clan double heat sink, two standard heat sinks, or four compact heat sinks), but is only counted as a "single" critical hit for all other purposes.
Hopefully, this helps.
Thank you,
- Herbert Beas
BattleTech
Catalyst Game Labs