There shouldn't be any problem with fielding the slaan in a TG bunker. As per the rules, the Slaan is in the 2nd rank and cannot be touched from the front so if he is placed with TG on either side as well he is immune to close combat attacks since no one can touch him (until the TG all die, of course!). Below is a diagram of a 20 TG unit with a slaan that illustrates how this works:
T= Temple Guard, S= Slaan (Slaan occupies 4 spaces due to base size but still counts as infantry and gets look out, sir roll from template attacks).
T T T T T
T S S T T
T S S T T
T T T T T
T T T T
Also, the TG actually end up being S5 since they have halberds - they are not allowed to go h/w & shield.