I thought I read somewhere in the 7th Ed book that a single model character within 5 inches of a unit can't be targetted but now I can't find that detail. I'd rather hide my skink priest near my skinks rather than in the unit incase it gets charged.
That was old rules, in fact I am pretty sure it was 6th ed not even 7th ed. These days they can be targetted. If the priest mostly has spells that don't need LoS, the safest place is directly behind a unit if you don't want to risk him getting in combat.
Something else to note is that if that priest is next to an infantry unit (I think it is within 3 or 6 inches), he gets a Look Out Sir roll of 4+.