One VM is just like the next. ESX doesn't care 32-bit 64-bit it's all the same. It's just a Virtual machine occupying CPU / Memory, that's it. There are no consequences to running 32-bit along side 64-bit.
They really aren't mixed, each VM runs in it's own memory / private space.
both 32 and 64bit VM's can exist together without any issues. You just need to ensure your Host ESX Server's CPU can support it.