For the most part, you can begin installing and configuring pfSense packages without worrying too much about the effects they will have on your pfSense system. Nonetheless, some caution is called for when installing packages on mission-critical systems. First, having some basic knowledge of the technologies underlying the packages you want to install is helpful. For example, it would be ill-advised to install routed (the Routing Information Protocol daemon) without having some basic knowledge of how dynamic protocols work in general and how RIPv1 and RIPv2 work (in this case, reviewing Chapter 10, Routing and Bridging, is a good starting point).
Beyond that, you should be mindful of the fact that installing additional packages may consume additional resources. Simple packages such as arping and cron can be installed on virtually any pfSense system without...