The poisoning sounds good, but what if you replaced it with something to the effect of:
Rat's Burrow
Permanent
Summoned when the card 'Rat' is played.
2: Sacrifice 3 Fleas to add +1|+1 to Rat and gain
.
If you play 2 Rats, then two Permanents are summoned, but if you activate a Burrow's ability, three Fleas are sacrificed to buff only the one Rat. You can't buy Rat's Burrow. I thought it might be a difference from all the poison and death effects. I've always dreamed of using a cyclical specialty permanent/creature combo to buff. It's just cool.