Wearing a sari with any tightly fitting cropped top is fine. You are not breaking any rules, because there are none. We honor the tradition of the sari by bringing it into our lives in a way that works for us. It's easy to hem your normal dressy t-shirt a few inches higher so that it just graces the top of your sari waistline. Perfect, comfy and involves no futzing with a measuring tape!
The choli can be matching or contrast style - it looks great either way. Working women wear the wildest and most enchanting combinations of petticoats, cholies and sarees.