You can change the grid by setup/Grid.
But I think it would be ideal to set the physical constraints so that the pairs will be routed with 8 mil gap all the way. (Corners tend to not be perfect on grid)
Hope this helps.
PCB Editor? The Slide function will normally be set to "gridless" then the traces will end up at the DRC spacing, regardless of the grid. You can change the grid from Setup>Grids, the Etch Grid controls the routing grid, The Constraint Manager Spacing rules will control the spacing between objects. Display>Measure, window select two objects will get you the Air Gap which is the spacing that PCB Editor cares about as far as the rules are concerned.
I'll look at the physical constraints and see if it works.