if you fill your front rank (of any unit) with characters & command group, then any additional characters go into the second rank.
You could even run a regular Saurus unit with full command (Champ, Standard, Musician) and 2 Scar Vets making up your front rank, and if your Slann joined that unit you could put him in the 2nd rank, just like with Temple Guard. However, once your Scar Vets start dying, the Slann will have to make way to the front of this unit.
I'm not sure what happens when you have Full Command + 1 Scar Vet + 1 regular Saurus. Since there isn't enough room for the Slann to fit in the front rank, I think he would still go in the 2nd rank until there are 2 regular Saurus in the front rank for him to displace.
With the Temple Guard then Slann can be in the 2nd rank if your front rank looks like this:
SMCVV
(S = standard, M = musician, C = champion, V = ScarVet)
But, if the Champion and one Scar Vet die then you would have:
SMVTT
And only 4 of the front rank would be Temple Guard, and 2 of them would be regular Temple Guard which would have to be filled in by the Slann... which would suck, because even after the last Scar Vet dies and you could have a front rank of 5 Temple Guard, you won't be able to move your Slann out of base-to-base with the opponent during a combat.
If, however, the Champion lives and both Scar Vets die, then you would end up with:
SMCTT
Which would be 5 Temple Guard so your Slann could continue to live happily in the 2nd rank.
So, if you're running Scar Vets in your TG unit ... keep your Champion alive!