Milestone rooted with the Universal Androot, can I install ROMs?
In short, your phone has a locked bootloader.  There is no way to unlock it (currently).  What does that mean to you?  Basically it means you can flash "ROMs", but you cannot flash a new kernel on the phone.  The kernel is what controls what clockspeed slots are available on your phone to over/underclock to.

For example, if someone made a Gingerbread ROM for the Milestone, you could flash it.  But you will always be stuck with the same kernel that originally came with your phone.

As far as I know, you should be able to use ROM manager to flash ROMs on your phone so long as it is rooted.  It's just that none of these ROMs will be able to change your kernel.