Andrew-
The conversion tools are about 95% accurate. They tend to be clunky because of the inherint differences betwen CSS and ACE (like the ability to apply a vip to all vlans vs. just one or a few). As well, there are some parts that do not port over like SSL because you need to import certificates and keys prior to configuration, the conversion tool has no awareness of linking between cert/key names between the two boxes.
ACE can share vlans with other devices, you just need to make sure there are no duplicate IPs. I.e. if you were to want to use the converter, then cut over to the ACE, you would need to add the vlans to the switch/plugin the ACE only after the CSS is offline so they don't fight over VIP IP ownership.
Regards,
Chris