Fios MI424WR with Airport Extreme Base AND 2 old AEx A1084s

The 2012 AirPort Extreme could be used with the Fios modem/router to create a wireless network. To do that, connect a permanently installed Ethernet cable from one of the LAN <--> ports on the Fios modem/router to the WAN "O" port on the AirPort Extreme and configure the AirPort Extreme to "create" a wireless network.

The AirPort Extreme could be located virtually anywhere that you want, since Ethernet cabling can be run 330 feet, or about 100 meters, with no loss.

The older A1084 versions of the AirPort Express do not have a setting to "extend a wireless network", so they could not be used if you plan to extend the network using a wireless connection between the AirPort Extreme and AirPort Express devices. You would need the A1264 version of the Express, or the current A1392 version of the Express if you want to have the Express devices extend the network wirelessly.

If you connect each A1084 AirPort Express back to the AirPort Extreme using a permanent, wired Ethernet cable connection, then the two A1084 Expresses could be configured to extend the network that way.

If you decide to leave the wireless "on" at the Fios router, you could use that signal for your "guests", if want to provide them with a separate network.

That would really be the only option if you want both AirPort Express devices to wirelessly extend the signal from the AirPort Extreme.

The older A1084 Expresses could still be used to "join" the network for use with AirPlay if needed.

Do I need to do anything to the Fios Router/Network A?

No

If I want both networks running does that screw anything up?

Wireless routers in very close proximity to each other can sometimes create interference issues between the networks. Good idea to physically separate the Fios and AirPort routers by at least 5-6 feet or a few meters or more to minimize the chances of interference issues.

Maybe I'm misreading the guest network capability here and I didn't do any modifications to any control panels Airport Utility just took it out of the box and let it do it's thing....

You are using the "main" network capability of the AirPort. The AirPort can also create an additional "Guest" network.....but only if.....it is connected to a simple modem. You don't have a simple modem.....you have a modem/router or gateway type of device.

Network B - guests and tenants. Network A - my stuff.

Got it, good plan.

Is it better to turn off wireless router wifi and use Airport express to run network

You lost me on that one. If you turn off the wireless on the Fios/Actiontec gateway, then you will only have wireless operating on one network.

A few responses I read on here made it sound like you COULDN'T in fact run two networks off the Fios/Airport set up because of DNS/MAC issues

One of the positives about a user to user forum is that you can get advice from any number of other users. At the same time, one of the negatives is that the advice is often, unfortunately not correct.

BUT it appears I can run both networks simultaneously and won't have any issues with folks logging in to Network B while I'm using Network A off my original router

Correct, as long as there are not too many users all trying to share the same Internet connection, which will slow down browsing for everyone.

base station one created Network B out of the box and base station 2 and 3 in bridge mode?).

That is correct. When you use AirPort Utility to set up Base Station 2 and 3, you really don't have to worry about setting up things like Bridge Mode. The utility will do this for you automatically.