You can have many untagged vlans - On different interfaces.. Pretty much all vlans are untagged to devices.. But no you can not run multiple untagged vlans on same physical interface.
If your connecting from your switch to an opt interface with no vlan setup - this is an untagged vlan.. You still need to set this up in your switches as some vlan ID, even if pfsense doesn't know about it.
But you have to have a switch that understands vlans to run multiple network on the same switch..
Do you have different physical switches your going to connect these different opt interfaces too? If so then sure what your doing is fine.
As to reasons of different interfaces for each vlan - the big reason to do this, is bandwidth... vlans on the same physical interface share bandwidth.. If I want vlan X and vlan Y to want full bandwidth of the physical interface, then yes I would put them on their own... And no you don't need to tag it, uplink just goes to switch that knows what vlan this traffic is, or a dumb switch that only devices on this network/vlan are going to be on.