Jay,
If I were to use a 6" sluggo, there are two methods to try. One....rig the sluggo on a 5/0 offset hook with no weight. Just cast out and twitch-pause-twitch. Two, use same hook but two feet ahead of sluggo attach a barrel swivel with a 1/2 to 3/4 oz. sliding weight in front of swivel ( carolina rig ). Weight should slide up and down line ( hole through center of sinker) This works well on a sandy bottom. Imitates a sand eel.
As for the bass assassins ( my favorite soft plastic).....you did not mention what size. I use a 4" black sea shad on a 3/8 oz. jighead fished with a slow erratic retrieve. 5" and larger bass assassins work well on a 3/4 oz. kalin ultimate jighead. Hope this helps. Doug B.