Armor is subtracted from damage, but damage cannot be reduced by more than 80%. As a result, if you have infinity armor you take 20% damage from everything (that doesn't pierce/ignore your armor). The Hardened Force Fields have 25% of the health of the non-hardened. Since they basically take 20% damage from everything they end up with more health. Specially, 25% more health. However, there are a number of ships which can wreck hardened force fields so use caution when using them. They work great if mixed in with normal force fields.
Stacked force fields work great. Damage is only ever applied to one force field. That FF takes damage and shrinks. Normally it shrinks so much whatever was being shot at isn't protected anymore and so the FF is left alone. When multiple FF protect one thing, only one of them is struck, typically the one physically closest to the attacker. Eventually that FF won't be protecting the target and then the next FF will start taking damage.
To abuse multiple FFs, you want damage FF to NOT be protected by other FFs. This is because when a FF is hit, all FFs under it cannot be repaired for a bit.