You can choose all stances at once indeed but with the risk not being able to max them all (or even at least two) completely afterwards. As the leveling up of the companion chocobo is pretty tiresome and costly plus the benefits are not so great either there is not many players out there with higher level companios to give us the exact amount of SPs you'll receive by the top level to calculate what you can or can't max out. I'm currently at level 3 after fairly extensive use of companinon and I've splitted the points between Heal and Attack so far ... Frankly - I'm not really that much bothered if I'll be unable to max out even one of the branches anyway.