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What colour shirt would go good with a burgundy blazer?
Depends really on the exact color of the blazer.
I would go with a light blue shirt with a burgundyish tie or a basic white shirt.
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What casual shoes go good with grey/black skinny jeans, wearing a moderately loose t-shirt.
Depends what color shirt you want to wear.
My suggestion is that black is hard to go wrong with.
Personally I have shoes in grey, black, dark brown, light tan, blue, and white.
Its just about picking the color that goes best with everything you are wearing.
For example a grey, black, white, blue checkered shirt with black pants. I could choose a shoe that is white, black, grey, or blue. Personally I would choose the blue to stand out more.
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how do you add a unique piece to your outfit should it be big like a shirt/pants or accessories
I happen to be a firm believer that over sized shirts and pants do not look good. The reasoning for this is that fitted clothing gives the appearance of looking fit. Even if you are overweight, less fabric = you not looking as big.
As for a unique accessories, they take awhile to find. It could be anything from a watch, to a bracelet, to a necklace, to rings.
Whatever you do, do not wear any of these:

It looks like something kids in the fifth grade wear.
I personally have a dog tag with lyrics to one of my favorite songs engraved on it, a bullet necklace, a four leaf clover charm, I have a few rings that are pretty simple and nothing to flashy.
I like to wear accessories that have meaning behind them because people always tend to ask me about them so I find its nice to have a story to go with it.