This is amazingly great advice. Don't go pre-"optimizing" by adding a bunch of needless network calls—you'll only end up with more instability if done naively. DO start from the beginning with clear interfaces and boundaries between concerns in your codebase. Design for services from day one. If you don't have discipline not to tightly couple objects together, split into multiple codebases (gems, modules, whatever) tested in isolation that can be deployed together until it's time to pull them apart.

A well designed class interface can be turned into an HTTP (or socket) API easily enough. By the time you need to, you'll probably have the resources to monitor, scale and maintain the additional service properly.