Can I load a ROM from a 7" Galaxy Tab onto a 10" Galaxy Tab? I have a handful of Galaxy Tab 10.1 (P7510) running Honeycomb, and a customer commercial application which runs beautifully on Froyo and Gingerbread, but has bugs on both Honeycomb and ICS.

Before I chuck thousands of pounds worth of hardware away and settle for running my app on smaller tablets, is there a sane downgrade path to 2.2/2.3 for the P7510? I have some 7" Galaxy Tabs if these would be a useful source of ROM images, for instance.
No, the key parts of the ROM would be the kernel and all the drivers for the hardware. The boot image is likely different too. These devices are not the same so you would end up, at the very least, "soft bricking" your device. Even when a device is basically the same, take all of the Galaxy S devices (vibrant, captivate, facinate, epic, i9000), all are basically the same device, but you cannot "cross flash" the ROMs between them. This is because the smallest change in hardware or boot image prevents this.