you're welcome to have whatever kind of shop you like :)
that said, you'll sell more if your shop is cohesive. I know no one likes to hear that, but it is just the nature of business. You don't have to only sell one item to be cohesive -- your branding can be as simple as having a photography style. Look at successful vintage shops -- they have a range of items by nature, but the good ones choose a particular era, and photograph their items in a similar manner.
The reason why cohesive shops tend sell more is because their items will attract the same type of customer. If your shop is cohesive, when someone wanders in because they like your necklace, they will probably also like your scarf, for example. Or if you focus on wedding items, a bride-to-be may find lots of things she likes that she wouldn't have thought to look for. It gives you multiple opportunities to sell to the same person, while also making your overall shop more appealing to that individual.
That said, having a variety of items allows you to "test the water" and find out what
the market is like for any given item. But over time, you'll most likely find that the majority of your sales come from the same few things over and over. Even if you don't narrow your offerings, your customers will likely do it for you ;)
