I love the idea of Fleas giving poison counters but dying often.
Keep the Rat exactly as it is, but use your revised idea:
Flea: 0 casting cost
creature, 0|1. Fleas come into play Infected. As long as there is at least one Flea on your field at the
end of your turn, gain 1 Poison counter.
(By making it a Death creature, you make Nightfall/Eclipse a bad idea to use with the Rat.)
That would give Adrenaline+Sewer Rat+(other Death-effect-related card) a great synergy.
