I'm not too worried about Mixape, Gyara is EVed in SDef and has more HP EVs than standard Bulky Gyarados.
As for Vileplume, you guys obviously don't get what I'm trying to say. Gyarados outstalls Vileplume (since it outstalls Venusaur...) Not to say Vileplume isn't a problem, because it is, but I just don't think it's as huge as you're acting like it is, because it really isn't. I know Gyarados can't hurt it, but it CAN stall it, much more than you'd think. Sleep Powder actually HELPS since I can PP stall you while I'm not using any attacks.
As for replacing Hippowdon for Tyranitar, my team would go from defensively strong to "run for your life! My only physical wall left is a utility pokemon!" kind of thing. Hippowdon isn't to set sandstorm (though it's VERY convenient), it's for a physical wall. Heracross would run me over with CB attacks if I can't outpredict it, and Tyranitar...gg. Stone Edge KOs everything without a rock resist.
As for MixMence ("As for" = win!), I've known that's been a problem for awhile now...Yeah, let's pick on the minor weaknesses instead of the major one! <--Yes, that was a joke. The awesome thing about MixMence is that it's very easy to wear out. The minute it sees Hippowdon (since that's usually my switch into Salamence), it thinks "DRACO METEOR" and Snorlax comes in, then Gyarados can come in and take Brick Break...it's pretty easy to outpredict. Again, I'm not saying it isn't a problem, since it's probably the biggest problem this team faces, but it can be defeated with the current crew.
Now, I think we've indentified our problems, to review:
- Subseeders that beat Celebi
- MixMence
- Stat-Uppers
And that's really it; I don't think MixApe is any problem. Gyarados is very easy to keep healthy. It would become a problem if I replaced Forretress though.
Speaking of Forretress, it sounds like everybody is yapping to replace it. Um, okay? Tentacruel looks good on paper, but it really does take a beating physical attackers, and Hippowdon cannot wall it all! I was thinking Cresselia...it would give me backup on MixApe, would toast MixMence, and would make a fool of our friend Vileplume. I lose much-needed resistances though...which is why I really don't want to replace it.
I'm going to make clear what I'm NOT replacing:
Hippowdon
Snorlax
Spiritomb
Gyarados
The other two are up for grabs.