What you need to worry about long before you exceed gcwr is gvwr or payload of your 1500 suburban. Since you weighed your burb loaded, subtract the scaled weight from the gvwr for the burb (should be posted on sticker on door jamb). The difference or remaining payload is what you can allow for tongue weight of whatever TT you decide on. Be aware that the tongue weight can be determined by multiplying the gvwr (not unloaded weight) of whatever TT you are interested in by 13%. For instance, a TT with a gvwr of 7000 lbs will have a tongue weight as high as 910 lbs. With 6 kids in the burb you will quickly exceed your payload.