The shield should click into place when you push it into position from the inside of the case. I always put in the board first, and loosely fasten the screws. Then I push the board back against the plate slightly and fasten the screws further. A bit of a gap won't hurt, but it should be minimal.
And yeah, use the spacers you have. Full atx boards have 9, mini-atx has 6 iirc.
Not sure what you're refering to with tabs.
