It doesn't look like there is good information on this. It's open to interpretation. It's never explicitly said you can use the thalaron weapon from being cloaked in the script and since this is a unique ship to 'Star Trek Nemesis' I don't think you can definitely say if it is or isn't possible. But, here's my answer.
LAFORGE: Captain, there is something else. I was reviewing the sensor logs. Look at this! When the Scimitar decloaked there was a momentary spike in the tertiary EM band. There! ...You're not going to believe this. It's thalaron.
This line implies that even though their cloak is perfect like Geordi says it's discharging thalaron radiation.
This line could also just be a plot device to give some foreshadowing and raise tension about the scary radiation.
Then again, it could mean that cloaking keeps this type of radiation in and stores it really well. Which makes me believe that the thalaron intermix that they show later in the movie near the climax would overload the cloak.
Though I bet they could charge it up a little before they needed to vent this radiation and drop cloak since the radiation didn't ever mess with the cloaking device once. This is also assuming this cloak follows the rules of other cloaking devices that need to be dropped for venting radiation build ups.