Yes. You can do all of the into the dark quests for each of the 2 grand companies. Into the dark and imperial devices are the only quests you can do for each of the three companies besides their own unique quests that they offer before you HAVE to choose a company to join. You'll know which quest it is you have to do to join when they tell you to visit each of the main cities and listen to the leaders make a speech. After that you'll have to join 1 company and then you can also buy your chocobo with seals earned. I highly recommend you purchase your chocobo license before any rank promotions or gear. Also, when you join 1 company, you throw away all the seals for the other 2, so before you commit to one you should spend all the seals you've earned for the other 2 first.