I'm not entirely sure if it is mandatory to deploy the reinforcement combat group(s) as soon as possible, and didn't seem to find this question raised on the forum. The commlink skill has an Obligatory label, but it says that you "can use the Request Reinforcements special skill" and the description on the first page also uses "can": "REQUEST REINFORCEMENTS The Active Player counts their Victory Points, and checks if they can request Reinforcements." But the Request Reinforcements skill uses "must" for pretty much everything: So which is it, do you have to deploy reinforcements as soon as you meet the conditions to do so, or can you stall their arrival until turn 3 even if you lose everything in your main force on turn one before you get to your first turn?
Until further notice the can on commlink skill makes it not obligatory to call in reinforcements as soon as they are available.