One of the ways IndiGo ensures low costs is by using a single aircraft fleet. That means quicker turnarounds, cheaper maintenance, cheaper training, cheaper spares, etc etc - the benefits are huge! Which in turn allows them to stick to their low cost banner.
I really think this ATR order is just another rumour which will never happen. IndiGo will be sticking to a single aircraft fleet for atleast another 15 years till the last A320 of the 280 ordered (100 + 180) is delivered in 2025. Post that they might even go in for wide bodied aircraft - you never know. But till that time, ATRs - highly unlikely.
Like you said, lets wait till the LeBouget Air show on Monday. Will be a real shocker if this happens!!