You do have a point but here is a counter point. She's a (I think true) vampire. Her vitality could run on different rules then humans do. What could weaken our life she on the otherhand could be immune to it for her. Like for instance. I could get my arm cut off in a dungeon. I would need some level of magic to keep said arm or even recover. She on the otherhand can drink blood and instantly recover. Toxins probably can just be expelled through natural process of waste release.
(I understand that you're suggesting a scenario where Rick is a mega-genius and intentionally creates a poisoned golden potion with the intent of tricking Allen into drinking it.)
Yeah, but the table has been flipped several times by now. Rick had no idea ahead of time that Kruz would be stuck in a prison with Allen, where neither could kill the on each other. And Rick had no idea about the interparty warfare was incoming, and so didn't prepare for it.
The author has been making a point of Rick having to adjust on the fly to the situation repeatedly changing from his expectations. This is just another case of that. Rick and Kruz both stuck, but... she does have that golden potion... which probably will trigger Allen's ability... and thus bust them both out of the prison.
(Note that this move is still high risk, since it's very likely that a powered up Allen will attack Kruz. I think that Rick makes the best bet and relies on Kruz (who would hate being sidelined anyway). Besides, if Rick dies, the dungeon and all it's monsters (including Kruz) dies with him.
In short, Rick intentionally triggering Allen's ability is a super high risk move, with no benefit (known ahead of time). Also getting into a situation where Allen could be tricked into drinking it is crazy unlikely, and not one that could (reasonably) be foreseen ahead of time.