Each member of your team can counter each other's weaknesses (ex. If your team consisted of a Normal-type (which is weak to Fighting) and a Flying-, Psychic- or Fairy-type, the ladder would cover the Normal-type's weakness.)
Mega Salamence is a Dragon/Flying-type and is weak to Ice(4x), Rock, Dragon, and Fairy.
Feraligatr is a pure-Water-type and is weak to Electric and Grass.
Marshadow is a Fighting/Ghost-type and is weak to Flying, Psychic, Ghost, and Fairy.
Staraptor is a Normal/Flying-type and is weak to Electric, Ice, and Rock.
So, knowing that, we can see that:
Mega Salamence can cover its own weakness to Dragon and Feraligatr's weakness to Grass.
Feraligatr can only cover Mega Salamence and Staraptor's weaknesses to Rock.
Marshadow can cover Mega Salamence and Staraptor's weaknesses to Ice and Rock as well as its own weaknesses to Psychic and Ghost.
Staraptor can only cover for Feraligatr's weakness to Grass.
This leaves you with Mega Salamence and Marshadow's weaknesses to Fairy, Marshadow's weakness to Flying, and Feraligatr and Staraptor's weaknesses to Electric not covered for in any way.
This chart I compiled shows a clearer picture of what I'm saying.
M. Salamence (Dragon/Flying)=ICE(Marshadow), Rock(Feraligatr or Marshadow), Dragon(M. Salamence), Fairy(x)
Feraligatr (Water)=Electric(x), Grass(M. Salamence, Staraptor)
Marshadow (Fighting/Ghost)=Flying(x), Psychic(Marshadow), Ghost(Marshadow), Fairy(x)
Staraptor (Normal/Flying)=Electric(x), Ice(Marshadow), Rock(Feraligatr or Marshadow)
With this knowledge and the knowledge of Alola, we can cover most of these weaknesses with Alolan Dugtrio (Ground/Steel), which is weak to Fire(Feraligatr and itself), Water(x), Fighting(M. Salamence or Staraptor), and Ground(Feraligatr).
This 5th team member can cover for Feraligatr and Staraptor's weaknesses to Electric, Mega Salamence and Staraptor's weaknesses to Ice and Rock, and Mega Salamence and Marshadow's weaknesses to Fairy.
Now all we have left is weaknesses to Water and Flying. The Electric-type can cover for these weakness and be your 6th team member (Jolteon, Xurkitree, Electivire, Lanturn, Emolga, Alolan Raichu, Vikavolt, Alolan Golem, Magnezone, Togedemaru, or Tapu Koko would be excellent choices).
[Note that these Pokémon listed under the 6th team member all have their weaknesses covered for by the rest of your team, including Alolan Dugtrio.]