See the supports-screens manifest document. Just set your target to 9 or higher and you can use xlarge. Any device below api level 9 will not be able to declare itself as xlarge and, therefore, will still be able to install your application . What is the drawback that users use for an Android tablet on which the version of Android (<3.0) is installed, which has not been developed taking into account the tables! I would not be too worried that tablet users are working with versions of android <3.0 (cellular), seeing your application in poor condition, since they are most accurately used for applications that look less perfect with this device configuration. I do not know if any tablets up to 3.0 are ready even to support the market?
Also see the dc parameter for the x-large parameter, which states that the default value for this will change depending on the version of the device, so it's best to declare it.
Everything in the manifest introduced in the new version of the platform that launches the application that installs the application will simply be ignored. This also applies to any xml layout attributes.
I would think that the screen size would be designed with screen resolution and density, im not sure if the OEM sets this at build time or if it is a calculation of the runtime. It would be nice if this were done based on the specifications of the device, but I have a sense of hard meaning (I could be wrong). In any case, I do not think that its backward compatibility (which means that it will be simply ignored) is in the manifest, since there will not be folders with specific resources.
Out of interest, why don't you want to set your goal above api level 8?