You can only wear one at a time - they can't be worn together. With or without a CIB, the only way you'd be authorised to wear the EIB is if you've tested for and were awarded it. If you havn't done this, you can't wear the EIB, even if you have a CIB.
Technically, you have your own discretion to wear whichever one you choose. However, if you've been awarded a CIB, and choose instead to wear an EIB you've earned, you can expect your chain of command to not be particularly thrilled about it.
NO. Army Regulation 670-1 specifies that only one badge from group one can be worn at a time. Group one consists of the EIB, CIB and CAB. Combat badges take precedence over skill badges. A soldier having earned both the EIB and CIB must wear the CIB. A soldier having earned both the CIB and CAB may choose which one to wear.
The EIB or CIB, you can wear after you reclass. The blue cord, however, may not be worn once you reclass, nor are the blue discs worn behind the infantry branch insignia worn with any other branch insignia.
With few exceptions, no. If you earn the EIB (or CIB) as an infantryman, then you reclass to another MOS, you can still wear it. If you earn the EIB as a non-11 series MOS, then reclass to infantry, you should be eligible to wear it. If you test for it as a non-11 Series, and never reclass to infantry, you can't wear it, even if you complete the testing. However, if you do complete the testing, you should be able to use the points towards promotion.
Yes. The CAB (along with the CIB and EIB) is a Category 1 badge, of which only one may be worn. The CMB is a Category 2 badge, and can be worn along with a Category 1 badge.
The Marine Corps doesn't have an award which is equivalent to the Army's EIB. You'd have to hold an 11 or 18 series MOS while serving in the Army and pass the EIB testing in order to be eligible to wear it.
No. The only time this is done is a matter of local policy, where the unit might dictate that those who were awarded the EIB previously (and thus aren't testing for it) wear it during the period in which EIB testing is taking place.
No. CAB is one of only three Category 1 badges (EIB and CIB are the other two). Being a higher category badge than any others, they are always worn above any other badges.
No.
No the CIB is a an Army award and not transferrable to the Marine Core dress uniform.
The Combat Infantryman Badge (CIB) is awarded to infantry personnel who have actively engaged in ground combat. It does not require expertise in marksmanship. Therefore, if you meet the qualification criteria for the CIB as an infantry soldier, you can wear the badge regardless of your marksmanship skills.