Can I replace only the smallest cog on my rear cassette? I currently have a 12-30t rear cassette which needs replacing.   While I'm at it, I really want to switch to an 11 tooth to get the extra 9% in top speed.  But the problem is, even the most expensive (e.g. Ultegra) component that will work with my bike only goes up to 11-28, and if I stick to Tiagra I'm stuck with 11-25.  For long steep hills on an off day, it's really nice to have the 30 tooth.

So my second thought was, the bottom 2 or 3 cogs usually aren't fixed to the cassette, they are individual cogs that can come off.  So maybe I could get a 12-30 replacement, and then a single 11t cog to replace the 12t or maybe I could even drop the 13t so the stepping goes 11-12-14.  This would be ideal as I use the 12t and 14t heavily but I virtually never use the 13t.

Would this work?

From what I can see, I would also need an 11t lock ring.  I'm having a difficult time finding both a lock ring and an 11t cog which claims to be compatible.  For instance, I found one cog which claimed to only work various cassettes which were already 11-xx and which were 2-3 grades above my Tiagra components.

Update: I found this PDF file on the Tiagra CS-4600 which suggests I need part Y1YF01000 ("Lock Ring for 11T Top Gear"), and Y1Z81100N ("Sprocket Wheel 11T (Built in spacer type)").  But I can't find anywhere (for instance Amazon or Google Shopping) to buy these part numbers or the corresponding parts.
You can replace the smallest cogs on cassette.
For example see Sheldon Brown's Building custom cassette page.


  For example, Shimano doesn't make any true "corncob" (one-tooth-jump)
  cassettes for time-trialists or flatland riders. In 7 speed, the
  closest is the J (13/14/15/16/17/19/21).
  
  If you remove the 21-tooth sprocket from a J, you can make it into a
  13-19 corncob by buying an 18 to put between the 17 and the 19.
  Alternately, you could make it into a 12-18 by removing the 19 and the
  21, and adding a 12 and an 18.


There is also about MTB &amp; Road compatibility:


  "Road" vs "Mountain" Hubs There is no interchangeability issue between
  "Road" vs "Mountain" cassettes and hubs as long as the number of
  sprockets matches. Although "Road" and "Mountain" hubs are no
  different as far as cassette fitting is concerned, they are different
  in terms of overall spacing. "Road" hubs generally use 130 mm spacing,
  while "mountain" hubs are 135 mm.
  
  "Mountain" hubs will likely be slightly better sealed against dirt and
  mud than "road" hubs, but this is rarely an issue in practice. The
  wider 135 mm spacing will generally result in a slightly stronger
  wheel due to reduced dishing of the spokes.