you need mixers in the rack - both for modulation (ideally a matrix mixer) and for sub-mixing audio before feeding to filters/effects etc, and for panning of mono signals if your final output is stereo
the simple way to judge if you need attenuators or an output module before hitting an external mixer or audio interface is to try it:
1 straight from final module - does it clip (distort) or sound bad in any other way? - if no stop - if yes continue
2 use attenuators - try attenuators in between the final module and the mixer/interface - same question as above - same results!
3 use an output module - if you need balanced outputs then get them - if you don't need balanced outputs, then the chances are that atttenuators and converter cables are all that is really needed
If you need a headphone output then get a headphone output - 2hp and ALM Busy Circuits are places to look for dedicated modules
1/4"->1/8" cables and Passive attenuators - are really cheap - and you almost definitely need them anyway - 2hp trim or pushermanproductions do a DIY 8 channel trimmer - or any other attenuator you already have to try
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities