Well, the thing about alien life is, it's alien ... Life on Earth hasn't worked out for vertebrates with more than 4 limbs. Different planets, different options (read "Wonderful Life" by Stephen J Gould for a dated but well presented argument that things were once, and could have been now, very different.)
A GB with 6 equal-ish limbs would need to be longer, just to allow for the issue you raise. Or the limbs might be shorter. Or the front limbs smaller/shorter, etc. Marauder648, do you think your artist would like the challenge of making a workable(ish) 6-limb variant? :)