When you say 'roof bars' do you mean it's got the rails running front to back on either side, or the parallel bars ready for the racks?
I've got roof rails and have Thule bars on there.
You've got to get the footpack specific to your vehicle (the Thule site and most sites selling them have little drop down menus to work that out). The feet grip onto the roof rails and attach to the bars running across (choice there is generally between the black rectangle ones, or 'aerodynamic' silver oval ones).
I've got a couple of Thule 591 carriers, which are basically just one step up from the basic. Advantage is basically that there is a dial at roof height to close the 'jaws' on the bike frame, and they lock there as well - just far far easier than a basic set I had before. To release the jaws you simply press a button. Has taken skinny 700c wheels and chunky 2.3 MTB tyres nae bother. And the jaws generally fit any bike frame I've put on there.
If you go for the rectangular bars the rack attaches with some metal 'loops' that are perfectly secure. On the aerodynamic bars you can use a 'T-track', which is basically a slot in the top of the bars into which a little 'gripper' goes and the racks slide on from the end.
Everything can be locked (bars to the rails; rack to the bars; bike to the rack), and you can even get a key kit that means you have one key for all of the locks (I haven't bothered with that). I can take some pics tonight if it helps?