Adding 802.11b/g AX(s) to multi-Airport Extreme 802.11n roaming network

Question

First, a thanks to folks like Tesserax and Bob for the great ideas on this forum -- the questions & answers I've read have helped me to date, though I still have a question!
My home wireless network today consists of:
One fifth-generation AEBS running in 802.11n only (5 GHz) / 802.11n only (2.4 GHz) mode, on channels 149 (automatic)/11 (automatic) -- provides DHCP services
One second-generation AEBS running in 802.11n only (5 GHz) mode, on channel 157 (automatic) -- bridge mode
There is a wired home network with Cat5e and a gigabit switch in the basement.  30 down/5 up DOCSIS 3.0 service through Time Warner Cable, upstream from the fifth-gen AEBS.  The AEBS units are connected in roaming mode via Ethernet.
Using the Airport Utility on my iOS devices, I'm getting "excellent" (59-65 Mb/s) connectivity when I am near either of the AEBS units.  It is a large brick house built in the 1920s, and while there are now no dead zones in the house, there are areas where I drop down to 10-15 Mb/s per the Airport Utility.  Unfortunately, the two AEBSs are at opposite ends of the house near external walls, one on first floor and one on second -- and the first floor rear AEBS needs to be where it is to serve a breakfast room and sunroom, while the home office AEBS needs to be where I have two Ethernet drops as the cable modem and switch are in the basement.
Before I added the fifth-gen AEBS, I had the older AEBS and two 802.11b/g Airport Express (AX) devices, also in roaming mode.  I wasn't thrilled with performance, and while performance is much better with the two AEBS model, I'm wondering about the benefits of redeploying my AX devices elsewhere in the house on the roaming network -- on the theory that I should be getting better 802.11g performance from a device close enough to get max speeds than I am from a farther-away 802.11n AEBS.
However, I am concerned about whether adding back in 802.11b/g devices on 2.4 GHz would restrict the performance of my 802.11n network?  I am assuming that might happen because I have 802.11n running on both frequencies... if I stepped back on the fifth-gen AEBS to 802.11n only (5 GHz) / 802.11b/g  [or a similar setting] on that device, will that help?  Or, am I running the risk of other cross-talk or interference -- or simply of my home network devices being befuddled by multiple access points in a single house?
Alternatively, I could try to replace my 802.11b/g AX units with 802.11n ones -- howeever, having just bought a new AEBS this week, that would encounter cross-talk with my Spousal Budgetary Compliance Unit (SBCU) and I could end up crashing the entire household....
Thoughts and ideas appreciated!

Answer 1

If you use the "b/g" AirPort Express devices in a roaming setup, any computers, iPads, etc in close proximity will connect at maximum "g" wireless speeds.
It's really not possible to predict the actual results in advance. The increased wireless coverage might tend to compensate for the lower speeds that would naturally occur due to signal drop off if you did not use the AirPort Express devices at all.
As long as the wireless access points are in a separate room, or separated by 25-30 feet or so from other access points, there should be no confusion that occurs with your laptop trying to connect. It will pick the strongest signal as it moves through the house.
I think the bottom line here is that this is one of those things where you just will not know unless you try. If the Ethernet cabling is already in place, it should not take long for you to find out how things are working.