I'm trying the Pomodoro technique. Sometimes, I need to make some strategic decisions. I need to consider some options and choose the best. These are creative tasks and I don't know exactly, how long they will take. Is it reasonable to make these tasks fit inside a Pomodoro grid? Does it make more sense to perform these tasks separately and return to pomodoro technique afterwards??
The Pomodoro technique can certainly help when you need to make a decision and/or be creative, but it may depend on the person.
People certainly can be creative under pressure and time constraints (for tips see this TedX talk). A professional writer for example won't have any problems using the pomodoro technique to do his/her work. Pomodoros help to focus on the creative process and not be distracted.
However, there are also many people who find it difficult to be creative under pressure. Their creativity is blocked or hindered because they know they have to be done soon, or it may take them 20 minutes to start being creative and then they only have a few minutes left. If you are one of those persons who find it difficult to be creative during a pomodoro, either do it more (with practice it will become easier), or just don't use the technique.
This being said, your question seems to be more about decision making than creativity. In my definition both concepts are related to each other, but not the same. When you have to make an important decision it's often best to sleep on it because that allows your unconcious to continue working on the problem and will lead to a better decision.
A good strategy for decision making is this: