well rifts are expensive cards (currently 8, could see 10 being reasonable) so i could see people trying to play a few at a time. the warplings damage relative to their numbers is exponential but that also means that its a slow start up from 0 and 2 respectively to 1+1 and 3+3 respectively ... 2+2+2 and 4+4+4 ... 3+3+3+3 and 5+5+5+5. a second rift would accelerate the rate, but most likely only after the slow startup period had occurred.
about the phase shield idea, i liked the concept, but i felt it may be too limiting, forcing people to put a shield card in the deck for non-shielding purposes if they wanted the warplings to dish out any damage. the next question would be what damage would the warplings do? if they can only do damage while the shield is active, they would have to be strong enough to warrant spending the quanta on the shields, the rifts and the warplings, but then it could easily slide into OP if the player is both nearly untouchable with the phase shield and hitting really hard with the warplings with the potential to stay protected by phase shield for a whopping 18 turns (assuming no perm control against the shields)