I think this works best if the little languages all share as much syntax and semantics as possible. A good example of this is OpenBSD's assortment of configura

I think this works best if the little languages all share as much syntax and semantics as possible. A good example of this is OpenBSD's assortment of configuration file syntaxen for OpenSMTPd, pf, httpd/relayd, etc.; each of those "little languages" differ considerably in their problem domains, but they all seem to share a vaguely-Tcl-ish syntax and have largely converged in semantics and typical structure. Another important consideration is that these languages are typically best when declarative as possible; if you can avoid Turing-completeness and stick entirely to something representing static data, then that's the ideal.