IF you have a slight gap in the wing sadde/wing join, but the wing and fuselage are both properly-aligned, don't sweat it. It's not a big deal. Just wrap a piece of plastic wrap or dropcloth around the wing where it will join the wing saddle. Then, apply a bead of clear silicone bathtub caulk around the wing saddle. Mount the wing and allow it all to cure. When the silicone is cured, remove the wing. The plasic wrap will easily peel from the silicone. Now you just use a very sharp single-edge razor blade to trim the silicone flush with the fuselage/wing fillet. You'll have a perfectly-fitting, nearly invisible joint.

If you fly with the gap, don't be at all concerned about "air pressure" getting inside the fuselage and causing problems with the wing bolts. That's just a non-starter. You won't get such a thing happening at all...whoever told you that didn't really know much about how things work. The only thing you need to worry about making sure the holes in the wing are properly-aligned with the holes in the hold-down plate. For best fit, you want zero gap between the wing and the hold-down plate. A gap there will allow the bolts to flex side-to-side, and could cause them to fatigue and weaken. When you have zero gap, the bolts will shear neatly if the wing is struck and tries to pivot...unless the bolts are much too big.