Lets start with this.
There is no such thing as a 'service feeder' there are 'service conductors' and there are feeder conductors but they are never combined.
I say that not to bust your chops but to help with the rest of your questions. It is important to use the terms the NEC uses when trying to learn NEC sections.
Service conductors only exist between the utility connection and the service disconnect or disconnects.
From article 100
Service Conductors. The conductors from the service point
to the service disconnecting means.
On the other hand feeders only exist between the service disconnects and the branch circuit breakers or fuses.
Feeder. All circuit conductors between the service equipment,
the source of a separately derived system, or other
power supply source and the final branch-circuit overcurrent
device.
If i understand this correctly.
From the meter the conductors can supply 2 to 6 switches disconnects.
I would forget about the meter as it does not exist to the NEC.
I would simply say that one set of service conductors can supply 1-6 service disconnects. (sometimes more for specific applications)
If this was a 100 amp service (a) can a 100 amp breaker on the bus be used
If you mean backfeeding a breaker to feed the panel that is still allowed assuming the breaker itself is not marked 'line and load' and the breaker has a hold down kit installed to keep it from coming off.
(b)if there is only 6 breakers to be used in the panel can it just be a mlo panel with the breakers as the 6 throws.
Yes, and that is often how large services are done. (At least in my area) You might have a 2000 amp MLO switchboard with up to six breakers in it each serving as a service disconnecting means.
Does a xfmr have the same requirements as the service feeders since its a separately derived system if it fed a. 100 amp panel?
No, you cannot run a single set of conductors directly from a transformer to a MLO panel with multiple overcurrent devices.
There needs to single over current device between the transformer and the panel or use a main breaker panel
However a single transformer can supply multiple over current devices directly if separate conductors are run from the transformer to each over current device.
In other words if you wanted to supply three over current devices directly from a transformer secondary you would have three sets of conductors in the transformer.
