Standard cable entry size on a typical ceiling speaker PC-1869S is 1.25mm2 max, although the new PC-1868 now has bare ends.
An F-2322C which uses phoenix connectors can accept up to 2.5mm2.

Depends on several things:How big is the amplifier (in watts)?

What are you ‘tapping’ the speakers at?

If we assume that the cable run is no more than 300M from amplifier to last speaker then cable can be 1mm standard 2 core flex. After that it is a simple case of maths – if you have a 60 watt amplifier you would generally want to leave 20% headroom, in an ideal case, which leaves you 48 Watts of load available. A typical example is shown below:

PC-1868 – (can be tapped at 6/3/2/1W) – at 6 Watts you can fit 48/6 = 8 x speakers , tapped at 3 watts you can fit 16 speakers and so on.

This example can be applied to any 100V loudspeaker, you just have to know what the taps of each speaker are.

And YES you can have many different types of speaker (ceiling, horn, wall, cabinet etc.) all on the same amplifier.

YES!!
If you are doing any kind of work with 100V line speaker systems an impedance meter is a vital part of your toolbox. It measures the load of a loudspeaker circuit and in this way prevents you from connecting a faulty load to the amplifier and causing damage to your amplifier. In the case of the ZM-104A this also produces a 1Khz tone, so you can walk around the installation and listen to every loudspeaker to see if it’s working. In most cases the speakers and cabling will be done prior to the amplifier being on site so the speaker lines can be fully tested and you can have a high level confidence that once the amplifier is connected everything will work.

Standard cable entry size on a typical ceiling speaker PC-1869S is 1.25mm2 max, although the new PC-1868 now has bare ends.
An F-2322C which uses phoenix connectors can accept up to 2.5mm2.