Shifting works like this, cause there seemed to be some question:
Shifting (regardless if its Formshift, normal, or Half) counts as an action in the same sense using Conversation does; the character is locked into the panel they it on, UNLESS they have Canto. However, Canto or not, they cannot attack the turn they shift unless there is an enemy adjacent to them after they shift. This means that unless that extra move matters, the first turn they shift does not take up a turn at all, providing they move first, then shift. The only penalty is lack of move, as was noted.
Using Items, however, does take up a full turn, cause its using an item, not shifting. Naturally, Canto lets people like Janaff or Vika able to move after it, but they can't ATTACK after using it. Basically, even if you allow Laguz Gems or Stones, the normal Laguz will be at a disadvantage relative to Laguz Royals. Since Laguz Royals can shift and transform on the same turn (and in Volug's case, Halfshift), I'm inclined to give them that 'Free Turn' to transform. They can't attack on this free turn, so we'll say its like "initiative, but enemy is ALWAYS out of range of attack" as a compromise, if you want to look at it that way.
Flipside, since Laguz items actually use a turn, I'd be inclined to NOT give the Laguz a free turn cause its a genuine turn usage. In game, Giffca may start with Laguz GEms and enough to sustain him the entire Endgame, but he's still worse than Caineghis cause Caineghis can attack on his first turn regardless, while Giffca cannot (E-3 aside, where every Laguz starts with a full Gauge)
Also, dude, Gareth starts with a Laguz STONE, not a Laguz Gem, so allowing him the Gem is silly! Granted, Dragon Transformation upkeeps are so good, a stone should have no problems lasting the entire fight.
In the end, though, forcing Formshift to take a turn is giving the Royals a disadvantage they do NOT have in game. The item usage, however, is a different story; whether you let them get a free turn to use it immediately, but force them to skip their next normal turn or whatever, an Item should steal the first offensive turn for that Laguz at very least, cause in game, that's exactly what it does. Formshift (and Halfshift), you're functionally shifted from the beginning to end.
EDIT: Also, I agree with Elfboy regarding Mantle. Its not like Goddess Protect on grounds that every PC used in Endgame (even the 2 you get later on) can break it with at least one attack (Defenses aside), where as Goddess Protect is breakable by 3 PCs per file (6 PCs total cast wise)