Are you planning to add a guide sleeve to the bracket? I solder a length of brass tube into the guide-post hole to function as a sleeve. The plastic guide-post inserts into the brass sleeve and is secured with a screw. I prefer to use guides fixed with screws, as opposed to clip-in guides because it is easier for me to adjust the height of the screw type.
I can't remember the details of the Ninco sprung guide. I suspect that there may be other guides that are better suited to your current application. Don't enlarge the hole yet. Whether you enlarge the hole depends on two things: the diameter of your brass tube, and the type of guide you will use. There are some guides that will not fit into the bracket hole without modification to the guide, bracket, or both. Some guides have "stops" that limit the maximum rotational angle of deflection of the guide blade. Some guides also have plastic material that surrounds the guide post; a type of shroud that strengthens the piece. It may be necessary to either remove or reduce the stops or shroud. Or you may need to reduce the outer diameter of the brass "ring" that physically holds the guide post. Determine if you need to reduce this outer diameter of the brass ring before you enlarge the hole in the brass ring.
A butt joint would probably be ok, just make sure that you center the brass tube over the hole in the bracket. A better solution is a brass tube that has an outer diameter that is a snug fit into the bracket hole, and has an inner diameter that sleeves over your guide post with a minimal amount of play. You can then solder the brass tube into the hole, adding strength and ensuring concentricity.
Of course, if your brass tube doesn't slip fit into the hole, you are back to the option of enlarging the hole in the bracket to accommodate your brass tube. If you don't need to reduce the outer dia. of the brass ring too much, or at all...go for it.
I have either a Slot.it or NSR screw fit guide to use. The tube that fits through the hole in the guide bracket is too fine for either guide post to fit in. The one that the guides fit in done fit through the hole in the bracket. I think I'll also need to trim a little off the front of the bracket to allow the guide to swivel freely.