Now, with your current Priority/Tank scheme, I can only seem to get it to work, so long as the number of mages stays constant in each raid. I used the following setup to make sure that the targets are marked in the correct order (remember, I have to have the raid icons in a specific order of star through square for kill order)
illidari blood lord (med tank) rank=15
illidari battle-mage (low? cc) rank=12
illidari assassin (med cc) rank=14
illidari archon (high tank) rank=17
This made sure that the Archon was our star (first kill order), blood lords were circle/diamond (2nd/3rd to kill), assassin was triangle (4th kill, but sheeped), and our battle-mages were moon/square (5th/6th and sheeped)
If the number of mages gets changed, the order will not work. I tried playing around with different ranks and priorities, but nothing seems to work for me quite right.
If I understand you correctly, you could make battle-mages higher priority than assasin(so that they will get sheep priority over the assasin), then just mirror the kill order in your sheep mark list, so that 1st sheep would be square, the second would be moon and the third sheep mark would be triangle.
This way, if you lose the assasin sheeper for example, only the battle mages will be marked with moon and square and assasin will still get the same symbol(as the triangle would be 4th in tank list)