What is and is not a valid routing is determined by the fare basis of the ticket. I've looked up a few CO TPA-BOS fares, and none of them seem to allow an IAH connection. Most of them have a routing section like
Code:
ROUTING 25 *** TRAVEL MUST NOT BE NONSTOP ***
25 FROM-TO TPA-EWR-BOS*
FROM-TO TPA-CLE-BOS*
FROM TPA-NYC-CLE-
OR FROM TPA-CLE-NYC-
OR FROM TPA-EWR-
OR FROM TPA-DTT/CLE-EWR-
OR FROM TPA-EWR/CLE-DTT-
OR FROM TPA-EWR/DTT-CLE-
TO BOS*
You might be able to get the routing you want by buying combining two round trips (such as TPA-IAH / IAH-BOS or TPA-EWR / EWR-BOS), but I'm not sure how much more this would cost. I would suggest using ITA and seaching
Code:
From: TPA:: CO IAH CO EWR CO
To: BOS:: CO EWR CO IAH CO
and seeing what comes up.
Good luck!