What a lovely boy! I like the look of the sheepskin lined girth.
If you can't return the bridle for a cob size, why not punch some holes in it at the point where it would fit him better? If you have a local leather or shoe repair shop the could do this. And then possibly shorten any long straps if needed so that they don't flap around.
To fit a girth, you should have several holes (at least 4 or so) left near the top of your billets after you fasten the girth tight enough for riding. What you want to avoid is the bulk of the girth buckle right under your leg where you grip.
He would be stunning in the colors that look good on palominos. Natural light colored leather, light blues, light greens, soft tans, golds, turquoise, soft shades of colors like pastels. I could so see Newmarket style leather on your boy :) or the light leather colors that are popular in western.
He has a cob style/delicate structure, so keeping his tack slim in style, not bulky will be a good look for him. The bridle you chose its an excellent profile for him. Definitely not a wide clunky style, this bridle design you have suits his face well. He would look lovely in one of those fitted style English pads or the smaller all purpose/English saddle pads. Stay away from the large boxish dressage style pads.
It's a lot like nuts and bolts - if the rider's nuts, the horse bolts! ~Nicholas Evans