ESX 3.5 and 4.1 can be comined with HA enviornment because The HA agent ( AAM ) is same in the versions 3.5 and 4.1 with 5 primary nodes and rest secondary nodes but Esxi 5 has HA agent (FDM ) each node is primary and both AAM and FDM agent are different. so you can't mix esxi 3.5 with esxi 5
Instead you can create seperate clusters for example Cluster A contains Esx 3.5 and Esxi 4.1 and Cluster B contains ESx 5
Thank you,
Deepak Negi
VCP 4.1, VCP 5