Generally a "surprise" round tends to be a decent compromise, which is a round in which the "ambushing" characters get half of a turn to act first and then initiative order comes into play.
So, if a character got 4AP every turn normally, then they'd have 2AP in the "surprise round," etc. After all the "ambushing" characters have taken their "surprise round" then the battle begins with initiative order. (Which means if the ambushing characters have higher initiative, they would act again before your party does).
