Do we have some "rule of thumb" to estimate the miscibility of metals?
Bottom line: there are no simple rules of thumb.
To demonstrate this, I will show a round robin of phase diagrams for fcc metals ($\ce{Ag}$, $\ce{Cu}$, $\ce{Au}$, $\ce{Ni}$) with no intermetallic compounds - just fcc solid and liquid.

$\ce{Ag}$-$\ce{Au}$, full mutual solubility, liquid and solid enthalpies of mixing pretty close:



$\ce{Ag}$-$\ce{Cu}$, with limited mutual solid solubility and a eutectic:



$\ce{Ag}$-$\ce{Ni}$, no solid solubility, a huge liquid miscibility gap:



$\ce{Au}$-$\ce{Cu}$, complete mutual solid solubility plus a eutectic:



$\ce{Au}$-$\ce{Ni}$, solid miscibility gap at lower temperatures, but complete solid miscibility above that, a slightly irregular liquidus:



$\ce{Cu}$-$\ce{Ni}$, solid miscibility gap at lower temperatures, pretty ideal liquidus:


Bottom line: even for nominally 'simple' combinations of near-ideal fcc metals coming up with a workable rule of thumb just doesn't work.