I was told that it wasn't possible to get a Chocobo solo because when you finally join a company, I think there are only 2 quests which are available and soloable that will give you seals, and it leaves you something like 400 short (can't remember exactly how much). But you can talk to the NPC in your Grand Company they ask for items and that's how I managed to get my last few seals. Then when I did do the 25 instance dungeon I could use it to rank up instantly.