You need to work more on the shape and form of your creature, what makes me say this is the shadows on its body (the shading); there's no actual apparent light source in your scene, what gives off the appearance of pillow shading. Focus more on getting the shadows on its body correct, make them hit its body where they need to (not just a slight shadow under the prominent features of your creature, such as under the jaw, or under its belly).
In order to achieve that rubbery look you need a stronger highlight, and also your creature needs to absorb some light onto its skin from its surroundings. You've got a really heavy colour around your creature, purple, use it in order to create reflection on areas where the light source doesn't directly the body.
You should also look up references and study why it looks the way it does, that would definitely give a boost.
For your third question, I'd say yes; it adds some more variety to the textures your creature holds.
For your last question it depends, which character would you give out as an example? They all, pretty much, have different art styles.
The only significant thing I can see that is related to all is the colour palette (which even then, had a lot of colours, so it wouldn't really make that much of a difference).
Another thing I want to mention is the colours. Right now the colour palette isn't very interesting (and even has some wasted colours. e.g that lighter olive green on his stomach, and light pink on the horns).
In order to make your palette more interesting I'd suggest adding hue-shifting, which I generally apply the logic of "hit by the light source = warmer colour and brighter, doesn't get hit by the light = colder colour and darker", there are obviously exceptions and such, but you shouldn't be too bothered with it now, as in your picture it appears to follow that logic.
Just a quick edit to demonstrate what I mean.
