Roof rack cross bars, do you have to have....?

the side rails in order for you to use cross bars? I was looking at cross bars on Carid.com the other day, and there seems to be some universal fit bars that are not adjustable front to rear, which I think is what the installation of the side bars get you. I could be COMPLETELY wrong here, but I am thinking that you remove the plastic insert that covers the holes for the rails, install the uni fit cross bar and then cut to size the plastic and snap it back into place.

So far I have piggy backed a few questions about lights and racks to other's questions and have gotten very little response. So I'll put this out there with a description of what I am trying to do. I am considering mounting a 40" light bar to a cross bar and wiring it into my dimmer switch so that it only works on high beam. From what I have been reading so far, I'm not sure what works.

Light. I have read a few websites on lights, the consensus on light bars is they work great if you are at low speed(<35mph). Above that speed, you out drive their capabilities. This is not my opinion, it is just what I have read, I have no real world experience with these light bars. For my philosophy of use, I need a light bar that will extend light forward at the 55mph to 75mph range....highway speed, that is useful light.

Theft. Have also been reading that the light bars are a popular theft item these days. So there is that problem to deal with.

Did a little more research on the subject today when it comes to roof racks. This is just my gut feelings and talking with a vendor of many brands of cross bars. It is very difficult to buy a set of cross bars that are guaranteed to fit the Mazda rails. Seems they are very hit or miss and unless you went to a store that had all variations available for you to try and fit into the rails, it is a crap shoot. It seems that they were pushing Yakima fairly hard as their best alternative to stock. Believe the price was over $400.

I have not looked into the light thing any further yet. I was looking at the roof of the car and no question drilling a hole to run power wires will be required. There are no moldings anywhere in the area along the windshield to run wires underneath. I need to further educate myself on lights before doing anything.