Is air to air refuelling possible at "cruising altitude"?
The main limitation is that the receiver needs to have enough excess thrust available to be able to precisely maintain formation on the tanker. Near the service ceiling, you typically don't have a lot of excess thrust, so refueling happens somewhat lower than that.  Since fighter generally have more excess thrust at a given altitude than a transport will, they can refuel at somewhat higher altitudes.
Other than wanting to have enough excess thrust available to accelerate forward in the event of a &quot;breakaway&quot; call, tankers don't have particular limitations that I'm aware of, though I've not flown those. (I'd assume being right at the limits of IAS / TAS isn't particularly comfortable for a tanker, so at least something below their service ceiling, but I don't know what that limit is.)
Pumps, rather than differential pressure, move the fuel, so that system is unlikely to drive any altitude constraints.
It's common to hear refueling happening in the mid-20's over the U.S.  How much that's driven by airspace availability vs by performance, I don't know.