Thrumming stone is better than you suggest. Every time it 'succeeds' in finding a relentless rat (which you may play for no cost), that new rat counts as a played spell as well, re-triggering the stone's ripple effect. We're not talking 3-5 rats, thrumming stone basically clears your whole deck of relentless rats and spews them into the field for free. so that's 56x 57/57 rats for 3 mana.
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As this is EDH, isn't your commander basically useless other than his mana affinity?
You need earthcraft for your squirrel nest. It's super pricy, but that doesn't matter in a proxy deck. See my squirrel deck: http://www.mtgvault.com/mrgifford/decks/squirrels/
Drop nimbus, add Kinsbaile IMO
Here's my take on the deck: http://www.mtgvault.com/mrgifford/decks/heroic/ As a bonus, my version is standard legal.
Heroic creatures is a good theme for a deck. I would focus on instants that target as many of your heroics as possible, like dauntless onslaught. That will help you trigger the abilities with fewer cards cast.
Deck edit.
you've got urza's power plant on there three times.
I went with the queen in mine. There are lots of little combos with making infinite slivers and gaining infinite mana and so on. Take a look if you have a minute http://www.mtgvault.com/ViewDeck.aspx?DeckID=363496