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Generally you'll need either an H2 or H3 for most super safety and FRT setups. Depending on the barrel length, caliber, and gas system- an H3 may have trouble cycling. But a minimum weight of H2 is recommended.
neither, this is an H3 buffer for a carbine length buffer tube
Its the same size as a standard carbine buffer.
I am not aware of any "pistol length buffers". However, if you are asking if it will work with a "pistol length barrel", then yes, absolutely. Assuming you have a carbine length buffer tube and spring. If you are using a "pistol" buffer tube that is carbine length, yes it will work. If you are using a pistol buffer tube say an inch shorter, an AR10 buffer in the 5.4oz flavor should work. If you are using a "shorty" pistol buffer tube 3.5" inches or less, definitely not. Those require proprietary buffer weights in very limited sizes/weights. Without being certain what buffer tube you have I cannot provide a definitive yes/no answer.
Yes h3 is a great choice for pistol length barrels