I'm looking to buy my first arduino kit. However I have a concern.
If my Arduino Mega is bought from one manufacturer, then can I buy shields or other parts from other manufacturers? Are there any incompatibility issues
Of course you can. In just the same way that you can buy a car from one manufacturer and bumper stickers from another.
That's the whole point of the Arduino platform - mix and match to make things -just like LEGO®.