picking the right sonos speakers as front speakers

Sonos doesn’t make a solution that allows a separate center speaker. And the Sonos Port is only stereo, it can’t handle anything higher. 

If you must go with separate front speakers for your home theater setup, you’ll need a Sonos Amp, which can interpret up to Dolby 5.1, and send a signal to your other Amp which is the surround speakers. But you’re losing that center speaker you’re currently using, in favor of a ‘faux/interpolated’ center channel, and you must use third party speakers, not Sonos, powered by that Amp.

Do you have any other SONOS units in addition to the PORT? 

I have no Sonos speakers at this time. I was 

thinking to connect the Sonos port to one of the exit from a Marantz amplifier to be able to expand the sound also to a pair of Sonos speakers while it is also  going through the non Sonos speakers and subwoofer that are already connected to the Marantz amplifier. 
I just wonder if two Era 100 would be enough , or should purchase two Era 300s.

The Sonos Port doesn’t ‘drive’ external speakers, but it can be grouped with other Sonos products.

It has a 75 ms delay on the analog line in, the Port would be a poor choice to take an input on video synced media streams from your Marantz amplifier. Anything you input to it would be delayed by that 75 ms, and end up being behind the Marantz connected speakers. In fact, any data at all, coming from the Marantz, would be delayed in comparison to speakers connected to the Marantz, no matter what type of data is sent. 

I did not think about the delay. What would you recommend that I can use a Sonos port then? Just to connect the Marantz to only Sonos speakers? Any other potential use?

thanks for all the answers 

Are you able to return the PORT? Potentially you could use it as a credit toward an AMP to drive the rear speakers. In the front, install one of the SONOS soundbars. I’m a premium type and would recommend the ARC ULTRA. In a SONOS based system there is no practical option to use the center ceiling speaker.

As suggested, the Sonos Port isn’t really a good device for a home theater solution. Sonos, like most other home theater vendors I’m semi-familiar with, really doesn’t like the use of external components in a home theater environment , other than speakers connected to the Amp. I concur with ​@buzz , his advice is the best if you want to go with Sonos. It will be hard to find anyone, I suspect, that will ‘fit’ with your Marantz. 

Also, you cannot use your current subwoofer in a SONOS soundbar based surround system.

Combining Sonos with a non-Sonos amp will not work for surround.