Assuming that the fire alarm system is:
- Wired as Class B
- A conventional or an addressable fire alarm system
- Not using voice evacuation
- Wired according to the installation sheets that come with the devices, and the installation manuals that come with the panels
If the system is wired Class A, then for redundancy purposes there has to be at least two conduits: one for feeding Class B and one for returning Class A.
Technically, I can't see a problem with including cables to all the zones to the field in one conduit. T-tapping though, is a different issue.
If the manufacturer allows T-tapping (usually, the only allow T-tapping for addressable Class B circuits), then for technical purposes, T-tapping will function correctly. To find out if the manufacturer allows T-tapping, you will have to look up the wiring diagrams in the manufacturer's installation sheets and installation manuals. These are the methods that have been tested and listed.
The other source of this information is the manufacturer's technical support team. When calling them, be prepared with the specific make and model of the fire alarm system, the devices that will be used, and anything else specific to the installation.
When talking to the Authority Having Jurisdiction (AHJ) or the technical support team, remember, they may think of the term T-tap differently from what you mean. In other words, be very specific about what is meant by a T-tap. T-tapping a circuit or wires is completely different from T-tapping a conduit. The wires make up the circuit; the conduit just carries the wires of the circuit.
The AHJ or technical support may be thinking about T-tapping wires when you are thinking about T-tapping conduit.
Beyond that, legally, it's up to the AHJs. Even the National Fire Protection Association, Inc. (the NFPA is a publishing house), in their definition of Acceptance and their definition of an AHJ, specify that the various AHJs have legal authority over the fire alarm system. In other words, the AHJs can have more stringent requirements than the absolute minimum requirements shown in the NFPA 72.