Cisco 1550 Mesh AP's and 3502's in Enterprise Mesh Mode Compatib
During the old mesh days when they had 'special code' then I would be suspect. However, if the 1550 and the 3500s are conecting to the same WLC and same code I dont think there will be any issues, although I havent tested it.
As for the 3500s, they do MESH mode.
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin