Mamoswine can learn endeavor which could be useful since you're running a suicide lead. Nature should be jolly/adamant.
Arcanine's moveset is okay, i don't think it's good in OU but it's your first team, the nature should be jolly/adamant as well.
Since Metagross is Adamant, Psychic will hit less, Ice Punch would be a good replacement.
Serperior is okay, you can also run defog over sub seeing your team has no hazard removal and you have a team with 2 mons 2x weak to stealth rocks (Gyara / Ark9)
On Hydre, U-Turn or focus blast might be better than charge beam, it doesn't really need that.
If you want a sweeper then Gyarados should be jolly/adamant, waterfall/power whip/ice fang provide the best coverage overall. But it can also run bounce as a stab move, just note it will take two turns to attack unless you run power herb.
Have fun!