Yes, they can be added together. The USPS often issues a 'make-up rate' stamp that can be added to the old first class rate to make the new rate. And the US has not devalued the postage stamp since the US Civil War.
The G make up rate stamp was issued in 1994. It has a value of 3 cents.
The D stamp would have a face value of 22 cents. There was a make up rate stamp with a value of 2 cents.
The price of a stamp went from 15 to 18 cents. So the face value of the B stamp is 18 cents. The B make up stamp would be worth 3 cents.
The G make up rate stamp can still be used for 3 cents of postage. The dove stamp was issued in 1995 when postage went from 29 cents to 32 cents.
42 cents is the 2008 rate. The rate went up on May 12 from 41 cents. You can check the USPS web site for a history of the rates and the current rates.
It is not a make up rate stamp. It was issued for the first class rate, which was 25 cents.
The previous rate was 29 cents and the new rate was 32. The make up stamp has a face value of 3 cents.
A G stamp has a face value of 32 cents. You would have to add 27 cents to mail a letter with it.
In the US, there is an advisory committee that reviews stamps. It includes members of the general public and stamp collectors. Along with the USPS, they come up with a set of possible subjects. The designs are either assigned to a USPS artist, or sometimes put out to bid. The head of the USPS makes the final decision after taking the recommendations into consideration.
There was no E make up rate stamp issued. The only stamp in the E series pictured the Earth and has a value of 25 cents.
I believe you are referring to the G Make up rate stamp. Issued in 1994 it has a face value of 3 cents.