My Team!

[Wolstenholme]

Hydreigon Lv. 67
Item: Razor Claw
Nature: Rash
Ability:
Moveset: Flamethrower, Crunch, Dragon Rush, Hyper voice

Larvesta Lv. 24
Nature: Jolly
Ability: Flame Body
Moveset: Ember, String shot, Leech life, Take down

Slowking Lv. 25
Nature: Quirky
Ability: Oblivious
Moveset: Headbutt, Ice Beam, Surf, Psyshock

Frost Rotom Lv. 24
Nature: Timid
Ability: Levitate
Moveset: Shadow ball, Shock wave, Toxic, Blizzard

Archen Lv. 25
Nature: Mild
Ability: Defeatist
Moveset: Pluck, Stone Edge, Agility, Quick Guard

Ferroseed Lv. 24
Nature: Impish
Ability: Iron Barbs
Moveset: Pin Missile, Gyro ball, Rollout, Curse

I have a couple of questions:
1. What difference does the nature make?
2. What items would you suggest?
3. Where should I go from here moves wise?
4. Do you think once fully-fledged (Evolved, leveled up and with a proper moveset) this will be a good team?

[Richard and Blaziken]

Natures raise one stat by 10%, while lowering another by 10%. Some natures do nothing at all (5 of them). You definitely want to pick natures that highlight a Pokemon's natural capabilities and movepool, while lowering a stat that it doesn't mind losing. See my guide here for a nature reference. Now onto the team.

Before I can recommend anything at all, I need to know a few things:

1. Are you planning on using this in any competitive capacity, or is this just for in-game?
2. Are you willing to breed for natures and egg moves? (by extension, are you willing to EV train, and even further, are you willing to IV breed? If you don't know what any of these things are, the guide I linked earlier will be of assistance :>)

With these things answered, I can better help you to determine where to go with your team. Keep in mind that competitive play and in-game play is much different, and requires different movesets. In-game, a heavy offensive is usually better than being defensive, but competitively, it depends on the Pokemon, your strategy, and the team around it. :>

[Wolstenholme]

Thank you for the help concerning nature, I can see why it makes a difference now.

As for the team I'd like to use it for in-game but also for competitive capacity (but not as much as I don't have compatible wi-fi for the game, so I'd only really be using it in the battle subway).
I don't mind breeding if it is really necessary but I'd rather not for entertainment purposes...

[quack98]

I'm going to go through each mon individually (I'm going to put the best nature, and then say if the one you've got is usable)

Hydreigon (Modest, rash is good as well tho)
Item: Life orb/Choice Specs

-> Dragon rush - It's physical, Hydreigon is a better special attacker, and rash nature is +Sp.Atk, maybe swap for Dragon Pulse
-> Crunch - See above, maybe swap for Dark Pulse
-> Flamethrower - Awesome coverage move
-> Hyper voice - A little pointless, as it doesn't add any coverage, maybe Focus blast.

Volcarona (Modest, Jolly is quite a bad nature, as it lowers sp.atk, ya might want to breed)
Item: Lum berry (stops status crippling)

I'm going to list a new moveset, as the one you've got is bad, but I'm assuming that's due to his low level and therefore lack of moves learnt

-> Quiver dance - AWESOME BOOSTING MOVE!!!
-> Fiery Dance - Nice STAB move, can boost Sp. Atk
-> Giga drain - Coverage, and I always think its nice to restore some health
-> Bug Buzz - STAB

Slowking - I'm sorry, I really can't help here because Ive never used him

Rotom - Is your heart set on frost? If so I'll edit in what I'd suggest, but I think rotom wash form would be better:

Rotom-W (Timid is the best, you did well there)
Item: Leftovers

Again, I'm gonna list a different moveset, because I've recommended a different form
-> Volt Switch/Thunderbolt - Depends if you want the switching or not
-> Hydro pump - Awesome STAB move
-> Thunder wave - Just to cripple opposing sweepers
-> Shadow ball - Good move, hence why you took it I suppose

Archeops (Adamant, Mild is just about usable, but not ideal)
Item: Flying gem

-> Stone edge - Good STAB move, you made a good choice there
-> Agility - He doesn't have time to set up, try EQ
-> Quick guard - I don't particularly like this move, but it is fairly useful for killing scizor
-> Pluck - Acrobatics is stronger, and with flying gem, no item (flying gem is used), STAB and insane attack stat, it's one guaranteed kill (almost)

Ferrothorn (Impish is great)
Item: Shed Shell

Im going to completely change this moveset as well, in favour of a more defensive one, which suits his stats more.
-> Stealth Rock - Keeps doing damage
-> Spikes - Awesome if you've the time to set up
-> Power whip - So you can do a bit of damage
-> Explosion - BOOM!!!

Edit: I was going to mention how awesome your sigs sprites are, they should all be shinies (if Game Freak hired you, a lot of shinies would be better)