i am playin a sword and shield char and i was wondering if i had to use a light shield to complete the set for bonus armour perk or if i could wear a heavy shield and still be able to get the bonus armour wit wearin full light armour..

I believe the armour set bonuses only apply to the four actual armour components - boots, curraiss, guantlets and helmet. So the shield will not affect those, however, a non-matching shield type will not recieve the armour set bonus if it is not matched. If it is, i.e. light armour + light shield, then the shield will get the appropriate bonuses.

I don't know because I've yet to try sword and shield, but considering the logic, this is probably right

I dual wield, and can get the benefits of the Well Fitted perk without a shield.

You don't need a light shield to get the light set bonus. However when you get the bonus it does also apply to the shield if it's of the right type, boosting its AR by 25%/50% so you will eventually want to use the appropriate shield to take advantage of that. On the other hand if you're planning to use smithing then it all becomes moot as that can boost any armor well above the armor cap without a shield.