Is there a balancing circuit? With a single cell it's easier as charging and discharging current only comes from the load and the charger. But when you have multiple cells there can be a balancing circuit which can affect the fuel gauge as some charge current is drained away.
There are tons of different fuel gauge ICs. Some are just dumb integrators (coulomb counter) that require your MCU to use an algorithm to convert it into a charge percentage. Others have a build in algorithm that estimate battery impedance and state of health using dynamic behavior of the battery voltage and optionally takes cell temperature into account.
Any fuel gauge IC that can work with your battery chemistry, voltage and current should work. You can do parametric searches.
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