AC Disconnect Ampacity - can fuse size be something other than 30 or 60 Amp?
Good news: you can go ahead with your plan

Unlike some utilities, FirstEnergy does not require solar/DER utility disconnects for single phase, self-contained-metering customers to be of a visible blade type, which means that your idea of using an AC disconnect box for this application is workable.  This is laid out in section 6.5 of the FirstEnergy Customer Interconnection Guide:


  6.5 DER installed at a single phase electric service location utilizing a self-contained company meter (usually 400A service size, or less), shall have a disconnect switch meeting the requirements of the NEC between the Company meter and the DER PoC. The switch shall be clearly marked, "Generator Disconnect Switch”, with permanent 3/8 inch or larger letters. The preferred location of the switch is outside the building in the immediate vicinity of the electric meter to facilitate access by Company personnel.


As to the headline question: yes, that's true as well

Given that you are getting a fusible disconnect, as non-fusible disconnects are also a thing, you can put smaller fuses in then the disconnect's amp rating, up to a limit (if you want to put a 1A fuse in a 60A disconnect, you'd need a set of fuse reducers for that).  Note that if you don't need to have overcurrent protection at that point, there's no reason not to get a non-fusible disconnect box vs. a fusible disconnect box as the non-fusible variety are somewhat cheaper and you don't have to worry about changing blown fuses in that case either.

Give me a sign, friend!

There is one thing you will have to do no matter what you buy, and that's go to your local trophy engraving shop and get them to engrave a plastic plate with the words "Generator Disconnect Switch" in letters 3/8" high or taller.  You'll then need to get a small hand-riveter and a couple of pop rivets and rivet the plate to the front cover of your disconnect box; this'll satisfy the permanent marking requirement imposed by FirstEnergy.