How do Logia powers work? PS: Warning, not sure whether parts should be considered spoiler.

It is said that Logia typed Devil Fruit Users cannot be hit without Haki. Yet while re-watching the Alabasta Arc, Ace was touched twice by Luffy. The first time when he flew inside the restaurant, Luffy hit Smoker and threw him up against Ace, both making a huge hole in the next couple of buildings. The second time was when Luffy was arm wrestling Ace, although this scene might not be canon though. So my question is, how was Luffy able to push back both Smoker and Ace when flying inside the restaurant, since he shouldn't be able to. Does this mean Logia users can "solidify" themselves or maybe only parts of themselves, kind of like coating with Haki? Also what would be the default form for a Logia user?
It is said that Logia typed Devil Fruit Users cannot be hit without Haki.

Luffy using water to hit Crocodile shows that Logia typed Devil Fruit's weakness can be used to hit without using Haki. So to harm a Logia is to exploit the element itself, using its properties to solidify the user and rendering them vulnerable to damage.
Source

Despite the fearsome power of Logia users, they are far from truly being invincible. Sneak attacks are more likely to work, as the user will not be ready to avoid them. However, with training, most Logia users learn to transform by reflex, eliminating the ability to strike them by conventional means. For example, Enel's ability has been trained to the point that he could reflexively change form while partially asleep using Mantra to predict his enemy's actions.
Inexperienced Logia users also tend to become seemingly overconfident, as they are unaccustomed to physically dodging attacks or receiving attacks, due to their powers. Thus, if their advantage is neutralized by some means, they then have to rely on their physical ability to dodge, a skill which they may or may not be proficient at.

Very high skill and training required to dodge any sudden attack. Physical contact is required to hit Logia user, so a well trained user can avoid  Haki-imbued attacks by changing the state.

Does this mean Logia users can &quot;solidify&quot; themselves or maybe only parts of themselves, kind of like coating with Haki?

Yes they can solidify and turn there part of body as they want. Crocodile hand turn into sand while he was holding Vivi. Aokiji using ice leg in One Piece Film: Z.
So it is clear that default form is human state and change according to requirement. And can be hit without using Haki.