Well i am a big fan of KISS (keep it simple stupid) 
so if your performance needs are low and you do not have a lot of scsi locks ( i recommend that for reading VMware KB: Analyzing SCSI Reservation conflicts on VMware Infrastructure 3.x, vSphere 4.x and vSphere 5.x ) then nothing stops you to use one datastore for simplicity reasons.
Regarding Veeam of course Veeam can use an ISCSI target as a repo and existing content can be imported via clicking Rescan Repository (Have a look at that KB KB1729: How to Move Veeam Backup & Replication Backup Files )
Regarding the LAMP, well if you are really concerned I/O wise, you have to measure it and then decide what you do. But to be honest, if you have 6 10k drives, you can not expect wonders.