I'm not quite sure what you are looking for, but I will attempt to answer your questions as asked.
->Is it possible in the US to have more than one rating at the same time for quite different planes like DC9/MD80 and B737 e.g.
Yes. In the USA, pilot certificates, once issued, are valid until suspended, revoked or surrendered. Ratings printed on the certificate remain in effect in perpetuity. Regulations define currency and qualification requirements for operating privileges to remain in effect. Rather than re-issue the certificate each time qualifications are re-established, logbook or training records are the proof of qualification for operating privileges. These records need not be carried on one's person. (except student pilots operating under a CFI's authorization.) It is important to note that the checkride administered to a PIC for operating a jet in air transport service is type specific and contains all the elements of an original issuance ATP/type rating/PIC check. The only real difference is who may administer the check. Only an FAA inspector, designated examiner or training center evaluator may issue original certificates. Approved company or training center check airmen may administer recurrent checks. Just because one has a type rating on their certificate does not mean they are current to act as PIC. It does reflect that they are qualified if all other requirements are met.
->Is there no clear Career-pattern?
No. Career patterns are determined by individual air carrier hiring and promotion policies. This can be affected by complex market forces like supply and demand or by any other rationale relied upon at the individual carrier. Union labor agreements usually set the policy if in effect. Not by national policy! Company seniority is the most common factor used by US airlines for consideration for upgrade. Smaller carriers and startups tend to appoint Captains according to thier own agenda. This may or may not include consideration of seniority. When you get hired at most major airlines, you are now the most junior (and lowest paid) co-pilot or FE in the company, regardless of your qualifications or experience. In other operations, the value of your experience may be of more immediate benefit to your bank account. This issue is one where you can find very strongly held opinions about which is right on both sides. Times have certainly changed in this regard in that there is less belief that an investment of time and work for low pay now will pay off later.
Is there any policy to make pilots a F/O before grading them up to the CPT-Ranking?
Yes, please see above. Direct-entry captains are hired by many carriers, sometimes causing resentment among the FOs who feel they should have gotten the nod. It's a sensitive subject in some companies. Most major US carriers have operated under the senority-based system since the early days of airlines. Startups are obviously exempt.
I'm not sure what relation your inclusion of accident references has to the issue of command appointment policy. Is there some correlation there that I failed to see?
Best regards,
Westhawk