Metagross
EVs : 140 HP/254 Atk/28 Def/88 Spd
Hammer Arm/Meteor Mash/Agility/Earthquake
Hammer Arm offers no coverage and is contradicting with Agility being used. ThunderPunch, Zen Headbutt, Ice Punch, or Explosion are all better options. ThunderPunch for Water-types, Zen Headbutt does decent damage to Rotom-A and Swampert, Ice Punch hits Gliscor and Zapdos, and Explosion could help open a sweep, etc. An EV spread of 60 HP / 252 Atk / 196 Speed would perform better in my opinion so that Metagross outruns Scarf Rotom-A and Scarfed neutral base 100's such as Scarf Flygon.
Feraligatr
EVs : 6 HP/252 Atk/252 Spd
Surf/Waterfall/Ice Punch/ X
Kingdra does the mixed sweeper role better in my opinion, I suggest focusing on a Swords Dance or Dragon Dance moveset. So, DD or SD should be used instead of Surf, and Earthquake in the last moveslot. Return and Aqua Jet could replace Ice Punch and Earthquake if you use SD, so you have perfect neutral coverage and decent priority.
Latias
EVs: 12 HP/252 DEF/204 Spd/42 S Atk
Ice Beam/Recover/Calm Mind/Psychic
This looks like an OU team, so she's banned. ;(
Magmortar
EVs : 252 S Atk/252 Spd/6 HP
Lava Plume/Thunderbolt/Focus Blast/ X
Magmortar is poor in OU, Infernape has better Speed and isn't Stealth Rock weak (same with Heatran).
Electivire
EVs : 252 Atk/252 Spd/6 HP
Ice punch/Thunder Punch/ Cross Chop/ Earthquake
Looks good, but I recommend a mixed set so Electivire has a stronger STAB move and isn't easily walled by many defensive Pokemon. It also gives you a decent mixed sweeper.
Electivire @ Expert Belt
36 Atk / 252 SpA / 220 Speed
Mild nature (+SpA, -Speed)
- Thunderbolt
- Hidden Power Ice / Hidden Power Grass
- Cross Chop
- Flamethrower
HP Ice is for Gliscor and Flygon (when switching-in), while Grass is for Swampert. Cross Chop defeats Tyranitar, Blissey, and Snorlax. Flamethrower damages Steel-types like Scizor and Forretress and Grass-types.
And for the final member, i'm planning on going with Adamant Torterra w/ Hammer Arm and Earthquake..
Torterra doesn't learn Hammer Arm by the way.
2. What items should I give them?
Life Orb for Metagross and Feraligatr, Choice Specs or Life Orb for Magmortar, and Expert Belt or Life Orb for Electivire.
3. Who should I use as lead?
Your last slot should be a lead that can set-up Stealth Rock (it helps damage Pokemon like Gyarados, and ensures many KOes), like Azelf, Metagross, Swampert, Jirachi, Infernape, Heatran, and so on.
Also, should I switch Feraligatr with something else like Milotic or Blastoise??
Not really, it isn't outclassed by them anyway. Gyarados could replace Feraligatr if you use a DD moveset though, since it has higher Atk, Speed, defenses, has Intimidate, and is immune to Spikes (but it's Stealth Rock weak).