1) BOA almost never increases credit limit for the student platinum card FYI.
2) Having a high utilization esp with only one credit card (at a limit of 600) is not good. You always want to stay below 30% of your card's limit. Ex. 600x30%=180
3) I really like CK but they are very misleading about their score (Transunion I believe). In my personal experience as well as what I've read, your actual score could be -50/+50 off.
4)What are you looking for in a credit card? It all depends on what you want in return! Do you mind paying an annual fee for the Amex GS? I personally would go with Chase Sapphire Preferred (with annual fee)/regular Sapphire (no annual fee) if you want mileage. If you plan on traveling abroad a lot, look for cards that have no foreign transaction fees. All Capital One cards don't have this fee. The Venture is a great card too! Cash back? I would recommend Chase freedom or Capital One Cash Back card.
With a salary of 55k Im sure someone will make an exception lol
Cash Back: Chase Freedom
/Amex Blue Cash
*/Capital One Cash
Mileage: Capital One Venture
Well Rounded Cards: Citi Thank You Premier
*I suggest going for the regular ones rather than the ones with the annual fees cause they are more lucrative and with your limited history, I think you'd have a better chance with them.