I assume that when presented with information like "don't use poison against a creature", the character wouldn't make that mistake, even if their player's brain is apparently out to lunch. I don't see how allowing someone to not waste their resources is "telling them how to run their characters"- but I'll be sure to ask my group how they feel about it next session.
The referee should be willing to remind the player of anything the character would know. If the player decides to do it anyway, that's their call.
But beware if the monster has a horny or bony carapace such as the Ankheg IIRC, then it counts as plate armor which scimitars have a -2 against.About weapon modifiers against different armors, I've only been in one game that uses those. I was a druid with a scimitar. Monsters that didn't obviously wear armor were considered AC 10 for weapon vs AC calculations. The scimitar has a +3 bonus against AC 10.
So +3 against most monsters that didn't obviously wear armor.
This was in the Slave Lords series of modules, so some people in armor. But still, +3 against most targets.
I expected this, but it was not how we played it. Aw well, it was the early 80s and we were all young.
The 1e DMG organization layout makes this some work to find it.
