How should a bra fit (especially the cups on small breasts)?

The most important thing to look for when trying on a bra is that the little piece of stretchy fabric between the two cups should lie flat against your chest. That indicates the correct band size. Since everyone's breasts are different and all stores make their bras a little bit different, you're just going to have to shop around for ones that fit perfectly. If there's a slight gap, it could be a matter of adjusting the straps a little differently. If that doesn't work, the slight gap is probably better than something that's super tight or squeezing your breasts anyway. I would go for a cup and band size that is as loose as possible while still 1) supporting your breasts from underneath and 2) the little piece of fabric I mentioned is flat against your chest. Not every store may have something for you.