Can 802.11n and 802.11ac devices share the same router?
Is it okay for 802.11n and 802.11ac devices to share the same router?
Firstly, it is okay to do so as long as your wireless router support both 802.11ac and 802.11n technology.
Secondly, all wireless routers have the ability to backward compatibility. For example, the Asus RT-AC66U wireless router can support the latest 802.11ac technology, but besides it also supports the previous technology, which are IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n.
https://www.asus.com/us/Networking/RTAC66U/specifications/
As of your question:
Will the overall speed for all devices be limited by the slowest device?
It depends on the device that you connect to your wireless router. For example, if you use an Apple iPad that supports 802.11n to connect to your wireless router, then your iPad can only reach the max speed of the N technology.
Please note that this situation does not impact other devices. At the same time when you connect another 802.11ac to this wireless router, you can get the speed of the AC technology.
So the answer to your question is NO, it does not work that way and you do not need to worry about getting the overall transfer speed of your wireless network decrease.
Yes they can all share the same AP (wireless router).
No, the presence of N devices will not force the whole network to use only N speeds. An AC AP will speak AC to an AC client, and speak N to an N client.
Please note that since N devices use slower signaling, they require more airtime to send the same amount of traffic compared to an AC client. So the more N clients you have in your mix, and the more active they are, the more they drag down the total aggregate throughput of the network. But it doesn't flip a switch and force everything to use N rates.
Please note that a device and an AP can only speak to each other using technologies they both support, so if the AP is N, AC devices will only be able to use N rates. Or if the devices are all N and only the AP supports AC, the AP will only be able to use N to talk to all those N devices. So it doesn't make sense to upgrade your AP to a new technology if you don't have any devices that support that new technology, and vice-versa.