Probably. But all of them count as a round. Makers just choose to select the standard length one as the one they use to say how many rounds will fit in a tube magazine. Much like a lever action carbine in .38 Special/.357 Magnum, which also has a tube mag - the manufacturer specs it with the shorter, ‘standard’ round.
Also, no, 2 3/4” is not 12ga Short. 12gs Short is mostly an obsolete measure, but they were 2”, 2 1/4” and 2 1/2”. 2 3/4” is and has been the standard for the past 40 years. Retail stores carry 2 3/4” and 3” and occasionally the 3.5”, but the 2.75” is what they will hand you if you go in and ask for some 12ga shotgun shells.
If that counts as a "round" then anything over 12" capacity will hold 6 rounds, and be banned. That means you'd be limited to 4 rounds of 2 3/4, or 3 rounds of 3 and 3 1/2 inch.