Is it OK the have a snug fit between the pads and rotor ? I haven't added the brake lines to the calipers yet so there may be a bit of fluid stopping the piston from going in any further ?
It should be. If everything else is as it should be you might want to throw a lock washer under the idler nut. But you should probably investigate why the idler arm mount is flexing enough under load to keep the idler from moving. Did you install the spacers correctly?