The ruling in the battle missions book is simply "Each player is allowed to pick up to three different models in their army as being 'specialists'..."
Nowhere does it say that walkers or vehicles cannot be specialists.
If the list is for a friendly kill team then yes, I'd agree with neferhet that Daemonic or psyker are good rules to go for. However, if it is for a tourny they usually have restrictions as to which rules you can have. I'd check with the event organiser to make sure. I only say this as I know the ones at Warhammer world don't let you have either rule.
Marbo is quite a good edition, but takes up a lot of points (for kill teams), cannot infiltrate onto the board and at the end of the day, is only one model. With every model able to aim at something different, it only takes one str 6 weapon to bring him down while everything else blows up what's left.
Not sure what you mean by "Prohibits to snapshot when charged" Neferhet. Are you saying relentless prevents you from snapshoting? Because that is slow and purposeful that does that, relentless by itself is fine, you can move, shoot, charge and fire overwatch.
“Insane? You dare to claim that we are insane? I find this very amusing. You see, there is no such thing as sanity. Why? Because the nature of sanity depends on the context the term is used. A man living in squalor may call the rich man insane for wasting food, while the rich man in return may call the poor insane for living off his waste. You slaughter thousands and squander the resources of planets on foolish incursions to please a corpse and his corrupted followers. And you have the nerve to call me insane? Oh poor misguided individual, pray to whatever deity you serve and hope he is merciful, for those who find themselves trapped in the spiders web have little else they can do”
Lord Nerthro of the ‘Heralds of Absurdity’ to what remained of an Imperial scouting party