Analog Practical Design
My question is it a good idea to set channel length (L) of all transistors same.

It depends on what you're designing and the CMOS technology you're using.

For logic circuits it is in general beneficial to use the same (minimum) channel length for all transistors. When you use the minimum channel length the transistors can be as small as possible saving you area and thus cost.

However, if you want logic circuits at a very low power and/or low leakage running at a low frequency then you could consider not using the minimum channel lenght. This might increase the size of your design but you should gain in power consumption (it will be lower).

For analog circuits you generally want and need to use transistors with a different length depending on how you're using a transistor. You already mentioned output impedance as a factor.

However some very small processes like 10 nm and below (with finFETs) might not even allow you to use a non-standard length. So in such a process you might have no choice.

then there may be problem while making layout of the design.

If the process allows for different lengths then this is not true, you can just make the transistor you want within practical limits of course. At least in my 20 years of CMOS IC design experience I have never seen a problem with this.