First of all: three ice weaknesses. So you better take out someone.
Suggestions if you wanna replace one of those (not all, which is why I'm suggesting multiple mega evolutions):
Nidoking -> Electivire (it's OK), Mega Manectric, Mega Scizor, Mega Aggron
Roserade -> Tentacruel, Mega Venusaur
Salamence -> Actually, this seems to be the best one to keep.
Below advice is assuming you are still using the listed Pokemon:
1. Nidoking: Item is Life Orb all the way. The only reason I'd see for your mixed set is so you can hit fairies and grass types super effectively with poison jab since they don't usually have high defense unless it's Chesnaught or Tangrowth. With that in mind, I would still remove Earthquake and add a coverage move that benefits from Sheer Force, like Ice Beam/Flamethrower/Thunderbolt. Or you can switch Poison Jab to Sludge Wave and Earthquake to something else, and move all those EV's to Special Attack. I would also remove Toxic Spikes for a coverage attack, or Protect for scouting, as Roserade already does it well.
2. Roserade: I think Energy Ball on top of Giga Drain is overkill. Switch that out for either Protect or a coverage move like Shadow Ball or Dazzling Gleam. Big Root -> Black Sludge for more consistent recovery.
3. Slowbro: I don't like to rely on hax items, but more power to you if it works. Sitrus Berry seems to be a good item if leftovers is reserved for Florges.
4. Char Y: Why 3 fire moves? Try Flamethrower, Focus Blast, Solarbeam. Plus pick one of Overheat/Protect/Air Slash.
5. Florges: Leftovers is a better item if you aren't using it on Slowbro already. 2 fairy moves is overkill. If it's singles, remove Dazzling Gleam and replace it with Protect or Wish. Petal Blizzard should be Energy Ball. Nature should be modest as advised below by PokeElf.
6. Salamence: Crunch is OK. Protect could be Dragon Dance, but I can see its usefulness to scout the enemy moves in an intimidate support set. Item should probably be Life Orb if you removed Nidoking, or Yache Berry to cut its ice weakness.