Megabus and Citylink Scotland
 

So, I will be travelling from Portree to Edinburgh using buses. From Portree to Inverness the Citylink Bus and from Inverness to Edinburgh the Mega Bus. I will be having a 20kg luggage with me. Do these buses have luggage compartments where I can place the luggage. I will be buying the tickets before hand.

Is there are reason you're taking the bus rather than the train? Long distance buses usually have compartments for luggage.

I submitted too soon. Yes, buses will have a compartment for your luggage. The driver may or may not lift it into the storage area for you. Usually they are very helpful, but some are not fond of lifting heavy bags. You should be there as they load your luggage, and, of course, when they take it off at end of your journey as they often like people to identify their luggage. It's still very casual. They will not likely take very large bags such as trunks or excessive luggage. It shouldn't be a problem for you.

Yes they do have luggage compartments under the bus

Because a using trains to arrive to edinburgh arrives late in Edinburgh, and the best is the bus.

No need to explain OP. Bus is a perfectly sensible way to travel in Scotland.