I recommend buying a speed cube like this (Da Yan) or another brand. They turn so much better than the regular Rubik's cubes.
Mirror cubes are solved the same way as other 3x3 cubes, you just have to change your mindset from colors to sizes and depths.
The Rubik's void is solved the same as a regular 3x3, with the exception of one parity that comes up.
2x2 cubes are very easy to solve if you can solve a 3x3.
The 4x4 cube is the hardest that i have tried to solve, be ready for the parity.
Larger odd numbered cubes are easier than the even numbered cubes, but generally the larger the cube the longer it takes.
The megaminx is not as daunting as it looks, most of the solving is the exact same. The only difference in the algorithms is the occasional extra step.
The Pyraminx doesn't require any 3x3 knowledge and can be quickly learned.
The square one is a very interesting cube, seeing how it doesn't keep its cubic form during the solve. This cube uses an entirely different notation and solving method.