First of all, I don't really mean mixing and combining as in that of Fairy Tail. Luffy's body will always be made out of rubber, but now if he eats the fruit it will also always be made out of fire. Now, since its always made out of both rubber and fire, those have to affect the other in some way. I doubt nothing special will happen to the body if it's made out of them both.The first and clearest response would be, he won't mix the both because this is not Fairy Tail. That's where people start sucking more elements and combine them.
And second, why he won't ever have poisonous attacks, it's because he hates people who fight with poison, it's too coward and his style is not to go about weakening his opponents with nothing more that head on beating. Poison is associated with the bad guys. Even Usopp doesn't have a poisonous attack, rather a sleeping gas or bad smell.
But I do agree with your second paragraph, about Luffy's fighting that is. Though it doesn't necessarily mean he will fight with poison, at least not intentionally. The poison might also only be produced from his body when he's using Gear Second, as a replacement for the steam.
But as I said earlier, I don't want Luffy to eat the fruit, nor do I think he will.
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