It's not necessary to remove any sail slugs from the mast track to reef, as long as the slug/slide above the reef cringle is high enough. You only have to get the bottom of the "slab" down far enough.
A careful sailmaker will ask you for measurements about the feeder slot and install the slugs/slides so you can get the reef grommet very close to the top of the boom. Since slugs and slides are usually spaced anywhere from 20 to 30" apart (+/- a few inches), it's not hard to accommodate a feeder slot and sail stop.
If you have a reef horn, put a "floppy ring" through the reef grommet. That's a ring on each side of the sail, connected by webbing or line. It's much easier to put the ring on the horn than to wrestle the grommet onto the horn.
If you're using a line rather than a hook for the fowrad reef line, do NOT put the line though the grommet, trapping the sail slides . Run it up one side of the sail to a ring or block at the reef, and back down on the SAME side.
See drawing below. (PS, I apologize for the crude drawing, I did it about 15 years ago with primitive software. It's ugly, but it gets my point across.)
View attachment 163246
Here's a drawing to show floppy rings. in Fig. 14. I don't remember where it's from so I can't give proper credit.