It is not necessary to get your chocobos to Rank B and above. In fact it is possible to gain a blue or green chocobo immediately after catching a great or good chocobo. However the chance of a blue or green chocobo appearing is very low. However it is in your best interest to upgrade their rank. (A blue and green chocobo are obtainable when Tifa is the lead of the party. Black and Gold, I'm pretty sure are not.)

Another thing to point out.

The best way of obtaining the carob nut from the Vlakorados early on is to use the enemy skill Bad Breath (obtainable from the Marlboro at Gaea's Cliff), which will inflict certain status ailments on the monster. Use steal on the Vlakorados, and you will gain a nut.