Upgrade from VCenter 5.5 to VCenter 6.0

Hi,

We currently have 4 hosts (Dell and IBM X hardware) with ESXi 5.5, VCops 5.8.3 and VCenter 5.5.

I am looking into upgrading my system to VCenter 6.0 however some f my hardware does not support ESXI 6.

I have the following questions:

1) Does it make sense to upgrade some of the hosts ESXI and leave the rest as they are? Can VCenter 6 work on environment with different ESXI versions?

2) Can I install VCenter 6.0 on hosts running ESXI 5.5? If possible, what new features can I use?

3) Is there any change needed to be done on VCops?

Thank you

Doron