I have been playing the game for around a month so still have lots to do on the original game. I see there are two game add ons. If I download them can I play them a long side the original game? Thanks for reading.

Yes, as soon as you rech required level to start main quest in addon, some npc will find you and it will give you first quest relating to addon. You can simply ignore that and finish main quest or all other quests you want or let everithing be and start addon quest, you decide.

EDIT: here, take some http://images.uesp.net/c/c4/Fishystick.jpg

Brillant, thanks for the reply.

Just beware that once you install dawngard random vampires will start attacking cities and can wipe out the population, including vendors quickly if you are not careful.