Okay, welcome to the site! It's always great to have new competitive battlers on the Database.
Now, the database is always eager to help give advice to new teams and such, but if you take everyone's advice, then you're going to end up with a completely different team that doesn't represent your battling style at all, so whatever you do to change your team, remember to add your own personal twist-- this is what makes battling so fun.
Alright, let's get started.
It's always good to have a bulky lead who has the opportunity to get off stealth rocks. Metagross's good physical endurance allows him to put up stealth rocks, and then get some damage in to start. You're EV spread and nature look good. What I would recommend is giving him leftovers, for they are more reliable in restoring health. Though it's true Metagross does tons of damage offensively and would benefit from this, Leftovers is consistent throughout the battle, giving you leftovers no matter how many times you have to set up rocks, switch in on a play, have an attack protected etc. The moveset looks good, but I would probably change Rock Slide. Though rock has good coverage on metagross's weaknesses of fire, --Ice, flying, bug are only a big threat when paired with fire and ground. Metagrosses biggest weaknesses are Gliscor, Landorus, Garchomp, Salamence, Dragonite etc. for fear of Earthquake hitting really hard. For this reason, I would encourage Ice Punch. Though you might not always outspeed the opponent, if you can predict a switch in then Ice Punch will 1HKO many of these threats. Ice punch is also more accurate and powerful than rock slide.
I love this guy, and use him frequently. He's quite powerful and is nearly unstoppable with a few quiver dances up, and has helped me make a comeback many times. I agree with the EV spread, once again, and whether you want Modest nature for extra power or Timid nature for extra immediate speed is really personal preference. When I use Volcarona, I use Giga Drain instead of Psychic, to cover Water and Rock weaknesses. If you choose to add Giga Drain, it pairs well with Life Orb because it heals off the recoil damage Life Orb does in exchange for the little extra power.
This guy looks great, a standard smogon set. Like JarJar said, you could replace Surf with Hydro Pump, but if you have the same luck with accuracy that I do and end up pounding your fist into the wall every time you use stone edge or Focus Blast, I think Surf will do just fine. Usually the extra 25 power doesn't cost you the game. This is also just a personal preference aspect to pokemon teams. It's also good that you recognized how important a rapid spinner was with your Volcarona's weakness to stealth rock.
It's good that you recognized that a healer would do your team well, ridding Volcarona/Starmie of a crippling paralysis, a burn on Salamence/Metagross, or poison on your Cofagrigus. However, there are more defensive options for healing. Here are some suggestions:
Chansey @ Eviolite
Trait: Natural Cure
EVs: 252 HP/ 252 Def/ 4 SpD
Chansey is one of the best special walls there is. Eviolite puts here stats through the roof, and allows her to come in on any special attack. With the access to Wish, Chansey can restore almost any pokemon back up to full health with a safe pass. Heal Bell does what all healers need, Toxic is a good stalling move for Chansey, while protect allows her wish to heal herself. Seismic Toss does 100 damage all the time, and is often able to help her 3HKO special sweepers without HP EVs. If you give her defensive EVs, Chansey can take most Physical attacks as well. If you don't like Chansey, you can always go with
Blissey @ Leftovers
Trait: Natural Cure
EVs: 252 HP/ 252 Def/ 4 SpA or 4 HP/ 252 Def/ 252 SpD/
Though Blissey can't offer the same defenses as Eviolite Chansey, she can access leftovers and hurt super slow Ferrothorn/Forretress with flamethrower, who would otherwise wall Chansey and apply pressure with entry hazards. Though I've tried both I still prefer Chansey for her massive defense, but it really, once again, comes down to personal preference. Both of these pokemon make excellent healers. But if you have your heart REALLY REALLY set on grass types then you can go with
Celebi @ Leftovers
Trait: Natural Cure
EVs: 252 HP/ 4 SpA/ 124 Def/ 128 SpD
Look familiar? This is your new and improved meganium. Same moveset, spread, and nature
with a better stats in every category. Though the typing may seem to have a lot of weaknesses, Celebi has good stats, and often finds an oppurtunity to Substitute.
I'm so happy you have this guy on your team. If you change his EVs to 252 HP/ 252 Def/ 4 Spe, Bold Nature, he would fore a Phenomenal core with Blissey/ Chansey. Chansey and Blissey's only weakness is fighting, and they happen to have extremely high Special Defense. Well, Cofagrigus is immune to fighting and has extremely high Physical Defense. Do you see what is happening? Synergy is starting to build. These two pokemon form a core that allow you to counter both pokemon's weaknesses with each other. Furthermore, Will-o-Wisp will cripple physical attackers, allowing Blissey and Chansey to take more Physical hits. But what about your precious wall being Toxic Stalled? No Problem, Chansey/Blissey can ring that lovely heal bell and make your team all better. Since you can heal this guy up, you might as well give him leftovers too.
An excellent physical attacker to top it all off. I can see you've balanced your team very well, two physical attackers, two special attackers, and two walls. This is a great Salamence, standard EVs. I would change his ability to Moxie though, allowing him to become an even more powerful sweeper, getting a +1 attack boost after every KO. Though Arial Ace is a good STAB move for Salamence, if you change his Nature to Naive or Hasty, you can utilize his special attack and give him Fire blast, allowing him to take down the few things that wall him after he has a D-Dance up.
Alright. Remember not to take all of my advice, and make your team your own.
Good luck in your future competitive battling!