R7500 and XR500 Comcast setup??

> With a R7500 modem/router and a XR500 router what is the best way to
> connect them so they're on the same subnet?

   An R7500 (or R<anything>, for that matter) is a router, not a
modem+router.  "Comcast" suggests that there's a cable modem (or
modem+router) of some kind involved somewhere.  A complete, accurate,
detailed equipment inventory might be a good first step.  Is your
"R7500" a plain/"v1" or a "v2" hardware version?  Look at the product
label.

   One way would be to configure all but one of your routers as wireless
access points.  For example, visit http://netgear.com/support , put in
your actual "R7500"(-like) model number, and look for Documentation.
Get the User Manual.  Look for something like "Use the Router as a
Wireless Access Point".

> [...] Would the XR500 be an AP?

   It could be.  Or the R7500 could be.  If you also have some other
router, then both the XR500 and R7500 could be.

> [...] Is it possible to create a bridge?

   I don't know what that means in this context.  What, exactly, would
"a bridge" be doing for you?

> [...] Do the routers need a hardline or wireless connection?

   As the picture in the manual shows, a wireless access point normally
uses a cable connection to the main router.  But other modes of
operation exist, too.  For example, look for "Use the Router in Bridge
Mode" in the R7500 manual.