bq27421 and bq27621

Those are good choices for smallest size and minimal power.  You can use the SHUTDOWN commands to put the gauges in a mode where they consume <1uA.  Even in normal mode and actively gauging, the bq27621 will only consume about 27uA.  It doesn't require a coulomb counter for it's algorithm and uses the Dynamic Voltage Correlation algorithm to estimate current based on the battery model.  The current and capacity registers (in mA and mAh) are therefore estimates, but give you a rough idea.  The SOC (%) accuracy is not affected by any error in the estimates, however, because SOC = RemainingCapacity / FulChargeCapacity and any error is canceled out from the numerator and denominator.  See the bq27621-G1 Quick Start Guide, but you could even use the bq27621-G1 with zero configuration if you just needed to display a rough SOC estimation on your product.  Of course, it has many tunable parameters should you need to dial in the accuracy.  One more note is that the bq27621-G1 contains three battery profiles inside it, and you can use commands to switch to a more appropriate one if the default doesn't match.  These three profiles are based on your charging voltage: 4.2V, 4.3V, and 4.35V.

The bq27421-G1 will take up the same space in your product, but adds a physical sense resistor to measure current.  This sense resistor is embedded within the IC, so no extra space is required.  The sense resistor improves current measurements and capacity estimations, but if you put it in shutdown mode it will be reinitializing the estimations ever time you exit shutdown.  The coulomb counter also requires more power when running.  If your currents are low, then the gauge will be in Sleep mode which reduces the power by turning off the coulomb counter.  You need to order the bq27421-G1A if you are working with a max 4.2V battery or bq27421-G1B if you use a max 4.3V or 4.35V battery. 

I would recommend you try the bq27621-G1 and see the note about working with very small or very large batteries on the last page of the Quick Start Guide.  You should scale your Design Capacity by 10 and tell the gauge it is 1500mAh or 2500mAh, for example.  Then divide any capacity registers you read out by 10.  This is to avoid trying to calculate SOC with numbers that are less than 100.  You could end up with SOC decrementing or inrementing by >1% if this happened.