  We present a scalable mixed-species Coulomb crystal clock based on the
$^1S_0$ $\leftrightarrow$ $^3P_0$ transition in $^{115}$In$^+$. $^{172}$Yb$^+$
ions are co-trapped and used for sympathetic cooling. Reproducible
interrogation conditions for mixed-species Coulomb crystals are ensured by a
conditional preparation sequence with permutation control. We demonstrate clock
operation with a 1In$^+$-3Yb$^+$ crystal, achieving a relative systematic
uncertainty of $2.5\times10^{-18}$ and a relative frequency instability of
$1.6\times10^{-15}/\sqrt{\tau/1\;s}$. We report on an absolute frequency
measurement with an uncertainty of $1.3\times10^{-16}$ and optical frequency
ratios relative to the $^{171}$Yb$^+$ (E3) and $^{87}$Sr clock transitions with
fractional uncertainties of $4.4$ and $4.7$ parts in 10$^{18}$, respectively.
The latter are among the most precise measurements of frequency ratios to date
and improve upon the previous uncertainty of the $^{115}$In$^+$/$^{87}$Sr ratio
by two orders of magnitude. We also demonstrate operation with four
$^{115}$In$^+$ clock ions, which reduces the instability to
$9.2\times10^{-16}/\sqrt{\tau/1\;s}$.
