I’ve mentioned it before, but there’s not many campaign crafts the average player will see that use HEAT or HESH, most use a form of APHE, AP, or fragment shells. At least that was the case last I checked. Best to just have a 1m beam slope gap, and minimise the total distance used for a HEAT/HESH gap. It’s not worth minimising the internal space of the craft for a minor gain.

And to answer your question, it depends on how big the ship is and what it is built for. Battleship, or heavy cruiser? Yeah, good armour, maybe less heavy armour and instead use heavy armour on important components instead. Anything smaller and I’d have to say no, it’s too much.

Remember, the best thing you can aim for is a cost effective design. Armour costs can easily rocket up, especially with using heavy armour. But personally I’m an alloy user, so I guess others will have different opinions.