Alright, first things first. The Linksys. I'll assume you know how to get into the router's setup with a web browser, but if you need clarification just let me know and I'll explain further. I reset my Linksys to the default settings and I will provide only the changes I made to make things easier.
In the Setup tab under Basic Setup:
Connection Type = Automatic Configuration - DHCP
Local IP Address = 10.0.1.201 (within 10.0.1.xxx but out of the TC's DHCP range. You don't have to but I prefer this)
Subnet Mask = 255.255.255.0
DHCP Server = Disable
Under Advanced Setup:
Operating Mode = Gateway
Under Wireless in Basic Wireless Settings:
Set your wireless mode (mine's mixed), SSID (make sure it's different than the TC's SSID), and Channel.
Under Wireless Security:
Choose the type of encryption you want. I chose WPA since I have some b/g devices that don't support WPA2.
You might want to set a router password under the Administration > Management tab as this would have been deleted during the reset. After that, you're gold! Save your changes, and disconnect the Linksys router. Now connect an ethernet cable from a LAN port on your Time Capsule into a LAN port (not the WAN port) of your Linksys. In the future, if you want to access your Linksys setup again, type in 10.0.1.201 (or whichever IP address you assigned to the router) from any computer connected to either the TC or the Linksys.
Now onto your Time Capsule. In AirPort Utility, enter Manual Setup and choose the Wireless tab:
Wireless Mode = Create a wireless network
Network Name should be different than the Linksys network
Radio Mode = 802.11n only (5 GHz)
Set up Wireless Security accordingly
That's it! You should now be running an 802.11n only 5 GHz network that will allow N clients to connect, and an 802.11b/g 2.4 GHz network for any non-N devices. Any devices connected to either network will be able to see computers, hard drives, printers, and any other network devices on either router. So to answer your question, yes b/g devices will be able to access your TC's HD as well as any USB devices connected to it.
If you need me to clarify any of these instructions or to supply screen caps of my configuration, just let me know. Hope I haven't confused you too much haha. Keep me informed of how this turns out.
~skorpien