To keep the pace fast, I would suggest going more granular than the timers you have, and move down to the round level. If you feel a player is taking too long to plan or think through their actions, tell them, to enforce the idea that they haven't got all day. Depending on the sensitivity of your players this could be as easy as "Character X hesitates this round, go character Y", or a simple reminder as Sandalfoot suggests "Clock's ticking".
With regards to the overall structure of the scene, a total round timer may or may not be appropriate. By itself, keeping the players from thinking too much every round and reacting more instinctively is probably enough to keep them on their toes, even if the number of rounds goes up. Depending on how the hostage situation is set up though, a limit like this might be appropriate (for example, if there are no bad guys with the hostages (not that the players know this) but there is a timed bomb strapped to the back of the door).
If the timer is not the way to go however, you should at least have a plan as to what people will do when the breach and clear goes down. Providing they are thinking rationally, the bad guys will have made a plan, however cursory, which you can use to add pressure to the situation without putting on an arbitrary time limit (eg. "once they recover from the initial shock, they will use the hostages as human shields"). The hostages will probably do something to, depending on their level of restraint. Having a group get up en masse and flee towards the exits in the middle of a pitched battle will add tension, if that is the way you want to go.
I would recommend also giving the players ample time to plan, with the information they have, before the fact. Let them know in character (they probably can figure it out themselves, but its good to be thorough) that they will be going into a fast-paced situation and won't have a lot of time to think once it starts. I wouldn't screw with them maliciously at this point, but if they make a mistake or something does not go according to plan it will also add to the sense of urgency, especially if their mistake is quite evident in the worsening of the situation (eg. maybe they planned that the hostages would be passive and wait to be rescued, but on sight of the cavalry, they instead start fighting with their captors most valiantly, getting injured or killed in the process).