Ok, this is a translation issue. I’ll try and explain.
1. The correct phrase for that would be “diminishing returns” rather than “penalties”. The word penalties would imply that you’re actually harming your ship by stacking resistances, and that’s not true. It is true that, for shields especially, the return on investment from adding tons of resists drops once you get past 50%, and then is further reduced if you can get them past 70%. That being said…you’re still getting some benefit there even if it doesn’t stack as high as it could, especially for a biweave that can regenerate a good amount of shields over time.
Hull is not totally like that, and is much much easier to stack. A heavy duty/deep plating hull reinforcement package adds both integrity and resistances to the hull. The integrity stacks additively but the resistances stack multiplicatively, and it can get downright ridiculous with the amount of highly resistant hull you can build up. Even if you leave the hull armor as lightweight (since hull armor has terrible return on investment when you look at mass vs integrity), a few HD/deep hull reinforcements can add hundreds of integrity at a minimal mass cost. That can save your ship in a pinch. A big ship like a corvette with lots of optional slots has no business skipping hull entirely. An FDL? Sure, skip the hull. There aren’t enough optional slots to do it properly there, so why bother in PvE?
2. “Cans” is not an Elite Dangerous word. You’re looking for “SCBs” or “Shield Cell Banks”. The thing about feedback rails is that the amount of SCB MJs cancelled depends on the amount of damage done by the rail or rails. You can’t install a single small feedback rail and expect to cancel a large class 7 or so SCB with even multiple shots. If I’m doing 128 DPS between my 3 fixed beams, that’s enough to out-DPS any SCB except a class 8, which is not commonly found on NPC ships. Canceling SCBs is far more useful when you’re flying a smaller ship that’s not packing two huge hardpoints and can’t out-DPS the SCB’s regeneration.
3. It’s totally fine to use either prismatics or biweaves on a corvette. It’s just a different philosophy of building shields. I like having high shield regeneration between engagements.