Sub......
You're 18 and have little to no track record so far. You have gotten you foot in the door and over $1K limit with AMEX. There are plenty of people around here that would kill for that option. You're interested in finances and learning about them by putting in your time here. That's a good thing but, you can't expect to take everything you know now and get a $10K limit right off the bat even though you have the knowledge to get past some of the hurdles. What's the rush to get all of these credit options?
Take the time to let lenders get to know you before rushing to pick up X more cards. Time and letting things mature will help more than adding a ton of plastic. Low limits will only get you more low limits if you try to go on a CC shopping spree. Put some effort into managing your new plastic and hit the CLI button as prescribed for each lender. Growing the lines you have acquired will better your chances down the road when you want to add more to your lineup.
Being concerned about AMEX giving you a chance at $1200 is kind of silly when they won't let most people even get a card. Do your 61/180 CLI's with them and give Disco something to compete with. I've got 20 years of experience with credit and Disco started me at $4500 because we just met each other and had to start somewhere. After I wined and dined them for a few dates every couple of months they loosened up more and more resulting in my limit today of $48K. AMEX took a chance around the same time to go out on a date and opened the door with $10K. After a few dates they upped it to $30K and with a little bit of pressure they managed to go to $50K. Well, I wanted them to do more and impress me with another jump so I asked for $150K and they shot me down and kicked me with a $700 increase which prompted a call to have them just take it back instead of keeping the odd CL amount.
Starting out and having envy is a double edged sword though. It's good to envy and learn about the hoops you will have to jump through to get where you want to go but, it's going to take time. Not everyone will have the same experiences when it comes to the same lenders. Some go into NFCU with a crap CR and get a $25K CC out of them and someone walks along with an 800 score and gets rejected completely. Same goes for other lenders but in the opposite direction where the 800 score gets them a $35K approval and they won't touch the other person with a ten foot pole.
You have a couple of good cornerstone cards between Disco and AMEX and can/will get where you want to be with some patience and persistence. When your first 90 days are up with Disco then hit the button. Since they recently made a change when they fixed an issue though the timing has changed a bit or was reset with the change/fix. Amex will grow quickly once you get past the first couple of CLI's with the whole 3X request methods. Then again being 18, student, and lower income it might take some pursuasion to get them to go above 10K/ea.
Like your other thread suggested you need more than 3 months to hit your bonus spend of $1K you may not really be ready to take on higher limits if you can't make the spend/payments in short order. Chasing a Platinum from AMEX might not be the best idea considering your $550 AF you would end up paying in the first couple of months and the high spend amount for the bonus points which is where a big chunk of value lies. If you can hit them for 100K bonus points for $5K spend in 3 months that's a tasty amount of points for future use. Don't put yourself in the place of needing to rationalize a big spend to hit a bonus and take on more debt than you really need to. Especially when you have to PIF with a charge card and/or use the Pay Over Time feature that's usually at 18% or so.
Starting Score: Ex08-732,Eq08-713,Tu08-717
