as for the title, can enemy heroes or unit champs challange the slann when he's in the second rank of a TG unit? It wouldn't make sense fluffwise, but i don't see rules which state that enemies cannot...
They can challenge ... But you don't have to accept. If you decline you will lose your BSB reroll if the Slann is also your BSB (per the FAQ). However, if you have another character or a unit champ you can accept with one of those models. Your opponent does not get to choose which of your models accepts the challenge.
That is why I like to stick a scar-vet in the unit as well. That way, I have the unit champ and the scar-vet to take those.