I'd suggest getting a deeper desk and order them the other way around (oldest left, not right). While a good selection of machines, the presentation looks very odd. Like just slapped on some random desk.
Nice collection. I would like to have both a 5150 and 5170 in my collection. I only have a 5162 (which is AT class and not an XT).
I think it depends on how you define generations, but I agree that I would not say there are more than four on that desk. The 5150 and 5160 would be one generation (8088). The 5170 is second generation (80286). The PS/1 is another generation (could be 80286, and is ISA, but these were designed as different than the AT and targeted at the Home market). The Thinkpad is a generation and a separate line.