Hi obrienj123,
If you are doing an interactive upgrade from esx 4.1 the installer wizard will offer the choice to upgrade or perform a fresh installation. If you upgrade, ESX partitions and configuration files are converted to be compatible with ESXi.
And yes you can have different versions of ESX on a cluster as far as the vCenter supports the versions and will be able to manage them. You should also make sure they are of the same processor type so it does not stop you from migrating the machines as its needed by DRS(If you are using it that is). Or you can also turn on EVC if you have servers with different processor families.