Gemini: The card as it is is already a cheap, durable striker, and summoning multiple would be just a bit OPpey. I'd up the cost a bit, or even make it like this:
Gemini: 3/4 (5 Aether quanta)
When you summon it, it becomes two Gemini.
Becomes a fun combo with Rewinds.
Pisces: This is unprecedented and cards like those not only can KILL EVERYTHING, they can also do this BEFORE YOU EVEN PLAY IT. This will be too strong, always.
Sagittarius: Ability is a bit strong, but it's a nice concept and I guess it can be lived with. And it's nice for Time while being too costly for anything else.