I don't see a reason to trust this account. Their other "scoop" was the release months of the specials and season 1, when there were already signs pointing to the quarters they'd be released. They could be right, but I imagine it's more likely that they're playing in to bronies frothing at the mouth in hopes of a sixth cast member.
If there's a sixth cast member, no way in hell will it be anything but a pony. The world of G5 just isn't set up for that.