@Long:
But ace isnt always fire or he'd burn down the white whale, same with akainu, kuzan freezing literally everything, sabo accidentally burning his hat, tashigi had to try and becone intangible in punk hazard
You're talking about turning into their element not their ability to not take physical harm.
To compare with Luffy again when he gain the fruit he became automatically immune to punches but actually stretching effectively was something he needed to train.
You could say the ace, tashigi and sabo moments were all played for comedy but oda has done worldbuilding through comedic situations plenty of times
Ace(presented as a season pirate) got shot by surprise and avoided damage in a serious scene but in a gag one he got crushed from Smoker being thrown at him. The first one gave him less time to react and yet it was the one he actually suffered no damage from.
Sabo when fighting the marines was immune to damage which was confirmed by Bastille telling everyone to stay back because he is a logia. And it would be weird to argue Sabo just skip training stage since not only did he get the fruit less than a hour earlier but he also mention how he is trying to dodge when he doesn't have to which pretty much mean he isn't activating anything himself.
Enel also avoided damage while sleeping and his whole point was that even if he wasn't doing anything it would be impossible to kill him. Except of course if you want to argue Enel felt the need to bluff against such an insignificant foe and despite the great pride he had shown in his power set until now.
Freely change original physical form sounds pretty definite
I'm sure I have mentioned Caribou choosing to take hits so I know they can choose to stay solid. The question was can you take give a logia damage by taking them off-guard I say no the protection is always on and they have to chose to turn it off. Else logia would not be that scary and they would not be that shocked to be able to get hit in the new world.
If it does need training I don't see anything pointing to that and I doubt we ever met or will ever meet someone who need it. even Sabo didn't need it less than hour later.