I would use the Arc, one Sub, and the two 300s as the Home Theater room, then the Fives as another room, and the Ones as another room as well, both for music purposes. I’d test which other room to add the other Sub to, it really depends on what rooms the speakers end up in. 

I’m wondering if you can use Arc and Fives in the front (with the Fives as Left and Right) and the 300s in the rear. 
Try that and let me know what you think!

No, external L/R speakers cannot be added to the front in a surround configuration.

The Arc remains a right, center, and left set of speakers, Sonos does not offer a way to use another pair of speaker as additional right and left front speakers in a Home Theater setup. There is no way to use any Sonos speaker as only a center speaker. 

I have 2 Fives and Sub.
Is there any way to add a center channel?
… and then get 2 300’s for rear surround

There is no way to use any Sonos speaker as only a center speaker. 

Can you use Fives for the front and 300’s for the rear (and no soundbar)?
I use the Fives and Sub or music, and trying to improve the TV watching. The Fives and Sub are used for the TV audio.

No, in that case you have no ‘front’ device that is interpreting the surround signal. That CPU is part of either the Sonos Amp, when you’re using two of another company’s speakers, or a Sonos soundbar, the Ray, the Beam, or the Arc. It is not built in to the Sonos Five. 

This means, if you’re currently using the Sonos Fives for your TV, that you’re only getting a stereo signal, which is all the device’s line in can handle, and the inherent 75ms delay that is on all Sonos analog line ins. 

It’s not using Line In - it’s connected to an Apple TV streaming device to get the audio.

And there is no easy way to toggle between Music and Theater?
Where Music = Fives + Sub  or maybe Fives + 300’s + Sub
and Theater = Arc + Sub + 300s

I’d expect that 2 Fives will provide better Music than an Arc.
2 Fives and 2 300’s would be nice for room-filling music, like for paries.

You can “Group” your FIVE’s with the surround “Room” when playing music. Grouping with TV audio is not usually satisfactory.