Well, Looking at your team comp, I'm going to try to guess what role each poke has, and what Seismitoad or Cloyster can offer this team.
Lets start off with Hydreigon
My best guess tells me that your Hydreigon is a special sweeper? Maybe a scarfed or Life orbed one? That seems like a nice solid poke to have, just be sure to take down any fairy that might wall him easily.
Next is Skarmory
Skarmary is a nice lead to set up rocks or spikes, and I believe he is also your only Physical wall so far on this team. That can be alright at times, but it can be trumped really easily (Trust me, the life of a thinking a charizard is mega charizard x, instead of mega charizard y is harsh ;~; )
Now to Gardevoir
I am guessing this is your mega, and your special wall as well. As long as you manage to get a alright moveset (Moonblast, Calm mind, and Hyper voice would be good examples) then your alright.
Another Special sweeper? So far the team looks REALLY weak to special walls like say, Clefable.
Anyway, it's a ok sweeper. It can be easily koed though by faster pokes (A Salamence could maybe 1 hit ko with earthquake, gain the moxie attack boost, and bam :/ )
Now Breloom can be a monster!
With the move spore, you can put to sleep big threats really early, and mach punch can be a amazing revenge killer, or just a good solid move with his ability technician. (Boosts the weakest moves. etc, etc.)
For Seismitoad or Cloyster
Well.... They are both good in there own respective rights, Cloyster with his Skill link ability paired with Icicle Spear, and Seismitoad for his good attack, hp, and elemental weakness.
However, I feel Seismitoad would be a better fit into this team. Seismitoad takes 1/2 damage from fire type moves, and none from electric, making him a ok switch in after skarmory, or when he is in danger, 1/2 damage from poison and steel type moves, making it another ok switch in after Gardevoir, and has a ok move pool with Knock-off, Drain-punch, Ice-punch, etc.
(I might have to try him out to be sure, but he seems like the more reliable choice since Hydreigon and cloyster share a x2 weakness to fighting)
Hope I helped :3