umm..."little" Tyrant here+immortal+cell=Mr.Badass?
and y do the upped one cost more with same everything?
Well, that takes 3 elements and would be pretty pointless in a rainbow, since you can just use Lava Destroyers for more damage and you don't have to pay as much or take a creature each turn. Forest Spectres would also probably be better since they are much cheaper and have only 2 less starting attack.
The upped doesn't die when it can't consume a creature, that's the difference.
Even so, I think it would be perfectly viable to make it +3/+1, since now it's just a nerfed Growth, or if you want to get into the fact that it's mono, think Fire Spectre.
To pick what to sacrifice, it should probably be the creature with the least hp, if they are even then just pick randomly among them. That'd give it a lot better synergy with FFQ.
The absorption is a maintenance cost in the unupped but an activation cost only in the upped? If so, you need to change the elite card description to reflect that. I think you need to keep the maintenance cost for elite card since you are already removing the "eat-or-die" condition.
Probably a bit too powerful--using otyugh as a reference, this starts with 4/6 attack (vs 0/3 or 0/5) and gains +2/+1 (vs +1/+1) with each consumed creature. So it starts stronger and gains faster, for only 3 additional starting quanta and 1 additional/turn?
It can only eat your creatures, like immolation.