Talking is a free action. Modify them to attack when you say a specific word.
By RAW, free actions can only be taken on your turn. Which you won't get until after the minion's turn, if they beat your initiative.
As for specific pre-combat orders, I'm afraid I have very little experience in dealing with mindless minions. The Dread Necro that I've built for myself is still just a character concept at the moment, as the campaign it's intended for hasn't begun. It's a tricky thing, because you can't really expect them to take the initiative (heh) on anything that is even a little vague. Modifying your orders to something like "Follow me.
Attack anyone that appears hostile to me. Attack anyone who attacks you." might work, but the ambiguity to the term "hostile" could cause problems. How does a mindless creature judge these things? They may ignore something that you would clearly define as hostile, while also attacking without reservation something that you consider completely harmless.
I'm sure someone smarter than myself can come up with a much better response for you.
Edit: Apparently I should have read the PHB
before posting, instead of after...
Excerpt from PHB, pg. 144:
In general, speaking is a free action that you can perform even when
it isn’t your turn. Some DMs may rule that a character can only
speak on his turn, or that a character can’t speak while flat-footed
(and thus can’t warn allies of a surprise threat until he has a chance
to act). Speaking more than few sentences is generally beyond the
limit of a free action; to communicate more information than that,
your DM may require that you take a move action or even a fullround
action.
Relevant bit is underlined. So, at the DM's discretion, "speaking" may, in fact, be a special immediate action, and McPoyo's solution would therefore be valid.