There's always at least one common, and it's probably Wario. He's a popular Mario character that transcended to have his own popular games, and lots of them too. He's in the same boat as Yoshi, and will likely be stocked to the same extent.
Rosalina was not common.
I doubt Wario will be, for nearly the same reasons.
We know his Super Mario amiibo is on the way, and it definitely his more iconic garb.
There are no commons anymore... I don't find it strange.
Maybe... but the thing is, I also ordered BOTH Smash poster sets on the same day (February 20, 2015) and yet Club Nintendo tells me that only the first set already shipped but not the second set. I was puzzled because I thought both would be shipped.
Can you still order the first set!?
I just heard a rumor that GameStop will only do online pre-orders for wave 4. Atleast that's what I've been hearing on an Amiibo Facebook group from a GameStop employee due to employees being accused of stealing people's pre-ordered Amiibo in store.
In other words... completely unreliable then.
Gamestop informs all of its employees via a digital newsletter weekly; if one knew, many would.
That said, an online internet social forum where one single person confirms an unlikely business model... is too far removed to even be considered.
Nintendo would need to issue a blanket statement to companies that the product cannot be preordered, and I don't even know if that is possible. I do know that the likelihood of all six amiibo-carrying-stores (Gamestop, BestBuy, Target, Toys R Us, Walmart, Amazon) deciding, suddenly at wave four, to now, individually, not want to keep the product in stores... is also nearly moot.
It may be a plausible (well, slightly MORE so) scenario on the basis alone of preorders being massively cancelled, and store exclusives being rather unbeneficial to each store (meaning, the cost of the amiibo per the shipment and difficulty to each store meant it was not worth it). However, we already know Walmart just took preorders for Gold Mario, and that other stores will carry Super Mario Line Amiibo. The employees at each store are not informed enough with amiibo to differentiate this being an allright scenario, but not the Smash amiibo (meaning, that the Smash amiibo cause problems, but the Super Mario are less likely too). For the majority of stores, I was actually for a long while informing the employees about amiibo, what they are, how the product line and ordering works, etc.
In short... they will be available to preorder, and there is no indication of when this will start.