does airport extreme 802.11a/c work with airport express 802.11n

Not sure what you mean by "work".

Can you provide some more details on what you are trying to accomplish?

Which AirPort will be the "main" router?

How will the "other" AirPort be configured on your network?

supposedly we will not experience any WiFi performance improvement except as for any new 802.11a/c devices

This is generally true, but the additional 2 antennas and higher antenna location in the new AirPort Extreme should provide a bit better coverage, and possibly a bit better speed for regular 802.11n devices.

If this is the case, will it be possible to use ou Airport Express 802.11n to extend the new 802.11a/c network to improve signal strength, not speed tho'?

You can use the AirPort Express to extend the signal provided by the new AirPort, but the Express will not extend "ac" wireless since it does not have that capability. The Express will extend an "n" wireless signal if that helps.

However, if you have an 802.11ac capable computer, like the new MacBook AIr....and....the Mac is located near the "n" AirPort Express, then the signal that the new Mac will receive will be an "n" level signal.

In other words, to get 802.11ac performance, everything.....the router and computer....must be 802.11ac capable.

The 802.11a/c device will connect to either the "main" or "guest" network.

In other words, an 802.11ac router supports ac, n, g, and b wireless devices. So, an ac device connects at ac speeds. An n device connects at n speeds, a g device connects at g speeds, etc.

Each device will connect according to it's own capabilities. You will only realize ac performance if you have an ac router and an ac computer though.

"You can use the AirPort Express to extend the signal provided by the new AirPort, but the Express will not extend "ac" wireless since it does not have that capability. The Express will extend an "n" wireless signal if that helps."

This is what I am trying to understand.

1. So AXTRM w/ sep SSID for 2.4 (b/g/n) - call it SSID1 - and 5GHz (ac) call it SSID2 - WiFi Explorer confirms AC mode on 5G.
2. Using an AXPRESS to extend - I have the option of SSID1 b/g/n *and* the 5GHz ac SSID2 in the pulldown (it sees both?) --> if I select SSID1, it also extends this same SSID in 5GHz (both showing b/g/n) >> so I now have 3 signals on SSID1 (XTRM 2.4G, XPRS 2.4G and XPRS 5G) b/g/n and one signal on SSID2 (XTRM 5G AC) ac
3. If I select the 5Ghz ac SSID2 to extend - it prompts me "do you also want to extend (the 2.4GHz SSID1)?" --> select yes and I now have 2 signals for SSID (XTRM 2.4G & XPRS 2.4G) b/g/n and 2 signals on SSID2 (XTRM 5G 802.11ac & XPRS 5G 802.11 a//n)

Is the 5G SSID2 actually being extended from the Airport Extreme to the Express?

The reason I bring this up is I am having a lot of issues since iOS8 came out (especially on iPhone 6). Everything I am reading is pointing to when people have 2 AP's set to use the same SSID. When you walk to a part of the house where 5G 802.11 ac signal marginalizes, the phone does not immediately hand over to the 5G 802.11 a/n Airport Express.

My thought was separate the SSID's but to me, if the Express can't extend 5Ghz from the Extreme, it's taking a step backwards. I am better off getting a more powerful router like an ASUS and going with one AP, drop the WiFi roaming altogether

Any thoughts?

**********************

update - seems the Express does extend the Extreme on 5Ghz- I was able to join the SSID2 on both AP's. Seems my problem has gone full circle now because I still have issues with the handoff between the two AP's (between 'ac' and 'n' on 5G).

The only way to keep the connection stable is to join the 2.4 SSID1. Speeds on 2.4 are not as good however. Pus it feels like an evolutionary step backwards.

In the latest firmware I think Apple did something special to prevent the AirPort Express from connecting to the new Extreme at 5 GHz. Because even within a few feet of the Extreme, the Express will not connect at 5 GHz. In fact, with a separate SSID, I can't even select the 5 GHz network on the Express!

The problem is that I'm not extending! I just want to receive the signal and send it across cable to computer connected. My old Extreme (3rd Gen) was much faster than this new one. What a step backwards if you ask me...

I was told by Apple, the best invesment would be to get 2 extremes. The price difference is not much seeing you can get a refurb extreme for $169 and the extreme is so much better. iPhone 6, iPad Air 2, MacBook air, plus I'm sure a lot of other devices will be adding AC as well. Better to protect your investment for long run seeing these devices are made to last for a long time

So basically pay double the price just to get back the speed that was lost by buying the Extreme in the first place? That's not good Apple. 😠 It's not even dropping support for the devices as much as it is doing something to the Extreme so that it is not standard compliant. If the 5GHz network was a "proper" 5GHz network, then I wouldn't have this issue in the first place.

Being forced to shell out $200 after already spending $200 just to get back the functionality that I had BEFORE spending all this money is ridiculous. Either there is something wrong with my Extreme, or AirPort is becoming an over-priced, restricted niche product that forces you to upgrade every couple years. Neither of which are welcoming thoughts.