Thoughtseize?

The main reason you run creatures in the board is because of your weakness to burn decks and other direct damage decks that ignore bridge.Ensnaring Bridge is great against most opponents but, against decks you need to race having creatures in the sideboard is essential to the 8 rack strategy. With your deck if your siding in creatures your going to be siding out your Ensnaring Bridges and your Smallpox at the same time (Mostly, There will be situations where you don't.) so that your creatures can go in as they please. I would probably suggest making changes to the creatures you have chosen to put in there though. (Specifically Nezumi Shortfang and Hunted Horror) to a creature with a little less utility but a more direct damage plan such as Nyxathid in situations where the bridge can't save you so you need to kill them asap.

With Pack Rat that isn't really the case though, he's fine to sit under the bridge and just pitch cards to so that when they finally break the bridge they have to deal with like 8 8/8 Pack Rats and you just sit there and giggle because your playing the rack and your a bad person. (Don't worry I do it all the time its ok)

My decks's overall strategy doesn't involve Pox so they are fairly different but as an example here is my deck's sideboard for a bit of reference. (My rats are mainboarded so you don't see them here.)

2x Bile Blight2x Darkblast3x Nyxathid2x Pithing Needle1x Shrieking Affliction1x Spellskite3x Surgical Extraction1x Syphon Life