help with rock area

I have cleaned off a rock area recently (no plants remaining) it was growing mainly gorse, wild heather and wild grass. However there are very fine roots still remaining in the soil and between the rocks which I cannot get at to remove. Would a weedkiller (ie roundup root killer) work or would you need to apply it to leaves to absorb it into the roots? Would it be unwise planting it without treating the area first? How should I proceed? If there is any chemical to treat area how long would I have to wait to plant?What plants would be suitable for this area, where soil would not be great, very dry? Most of the sorrounding area is still quite wild so would need something with good cover to help reduce area for seeds blowing in, and something which might also give a bit of colour (if that not asking too much!