I run a similar deck, but I use a Gourgeist (XY) line as my initial damage spreader for several reasons:
Pumpkaboo (XY) can Confuse (for just P energy cost) the Defending Pokémon which can buy some valuable time vs. a Pineco whose attack (for CC energy cost) is really not that effective.
Gourgeist is an attacker and not just a bench-warmer. Gourgeist has 2 attacks, both of which can not be countered by an opponent's Ability-stopping action. The first attack drops 2 damage counters (for just P energy cost) onto each opponent's Poké, and Gourgeist's second attack, Spirit Scream (for just PP energy cost), places damage counters onto the Defending and itself until there is only 10 HP remaining (I really miss Ht Lasers...). This effect is fantastic for setting-up Meowstic's Ear Influence attack.
Since Gourgeist and Pumpkaboo are P Poké, there is immediate synergy with the deck's primary P Poké and other key resources (e.g., Mystery energy and Dimension Valley), unlike Forretress.
Also, assuming the 4 SSUs are there for recovering Forretress (for reuse), Shaymins (for reuse or prize denial) and/or wounded Gallades, you could be Item-locked and not be able to use the SSUs at all. On the other hand, Gourgeist can continue to be useful despite the Item-lock.
Lastly, an opponent can Lysandre your Forretress and "hide" behind it if you can't remove it from the active position.
By the way, 1 (only?) key advantage I do see that Forretress has over Gourgeist is that it does not require any energy attachment to be useful. So, you need only focus on attaching energy onto the Gallades.
Running a 2-2 'Bats line would offer more advantages vs. Forretress. You could even use Goldbat's Swope attack to inflict 10 damage onto each of your opponent's Poké for no energy cost when a Dimension Valley is in play.
Other thoughts/comments for your consideration:
Level Ball is soon to be released again with the next set. Incorporating a few LBs would be helpful in getting your bench setup fast.
Despite the fact that Mystery energy would be a primary target for opponents' E Hammers (of the players' decks I've reviewed, most run either 1 or 2), the value of free retreat can be very critical at times. Secondly, you can mitigate this concern by running Bunnelby (PRC 121) whose attack can recover discarded resources.
IMO, you will be playing from "behind in prizes" early-mid game until you can get M Gallade EX setup given the "low" HP of most of your Poké.
Should your opponent Item-lock you early game, your deck operations will be greatly imparied, IMO.
Given the above comments/thoughts, following are some suggested deck changes for your consideration:
-2 Pineco
-2 Forretress
-1 Shaymin EX (other card-draw support can be used)
-1 Pokémon Fan Club (1 should be sufficient with inclusion of Level Balls)
-4 Super Scoop Up (only 50% chance of success or 0% chance should you be Item-locked (btw, more Item-locking capability coming with the next set too))
-2 Trainer's Mail (not needed as much with improved card-draw support)
-1 Startling Megaphone (1 should be sufficient, IMO)
-1 Switch (not needed as much with inclusion of Mystery energy)
+3 Pumpkaboo (or Zubat)
+3 Gourgeist (or Golbat) (or +2 and +1 Bunnelby)
+3 Level Ball
+2 Shauna (more card draw)
+3 Mystery energy
With the improved card-draw and setup speed, I would not recommend incorporating more than 1 Hoopa EX (which would need to be powered-up manually 3 turns... SLOW).
Lastly, should you consider using Bats, also consider incorporating Wobbuffet (PHF) whose Psychic Assault attack can potentially inflict lots of damage onto the Defending Poké. Secondly, its Bide Barricade Ability nullifies non-Psychic Poké Abilities.
I hope you find these comments helpful.
