Hi!
If you were to use rouse after Bahamut, you would be sitting with rouse on cd for 40 seconds(bahamut ends at about 1 min 30-40 sec) which is more than half of its cool down. It is highly likely that you will lose another cast of rouse. Also, contagion is used together with Bahamut, thus rouse + dwt won't be up with contagion unless you will till 2 mins 15-20 sec(your 3rd rouse would be up and used by then if you used it before Bahamut) .