Q1) theoretically, for best practices, you should have the hubs and the Wi-Fi source each 3 m away from each other. These are Omnidirectional antennas, so when they are too close together, you can get some interference. It doesn’t usually result in messages actually lost, but they may have to be resent a couple of times to get through and of course very strong Wi-Fi can drown out Zigbee.
But as @jkp said, if what you have now is working for you, there’s no real reason to change.
Q2) absolutely, stronger radios can create more interference.
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If you’re interested in tips on best practices, read note 11 in the following thread, then go up to the top of thread, and read the whole thing, and you’ll understand more about the potential issues. I’ll link directly to that post.
A Guide to Wireless Range & Repeaters - #11 by JDRoberts
But again, as @JKP said, if you aren’t noticing any actual problems, you probably don’t have to worry about it. 