I've seen that ide->scsi is possible ( http://www.sanbarrow.com/ide2scsi.html )but scsi->ide is more tricky. You should probably create a new ide virtual disk and then clone (with ghost as example) the scsi partition onto it.
For creating a new VM with IDE disks you've to choose custom and after some steps you'll be asked for a virtual disk type. There you can select SCSI or IDE.
why it has to be greater then 8Gb? Why this
It's not a limitation... it's that the geometry tables change below 8Gb; so you'd need to know the tables and/or formulae. At that point, it's much easier to just create a new disk of the type/size you want and throw away all except the descriptor-file, which will contain the values you will need.