Regarding using the Mother 32 with another VCO, I assume you're meaning using the 'external audio' input jack on the top left of the patchbay. Just note that patching there inserts the signal before the VCF and the VCA in the module, so unless you're looking to turn the Mother 32 into a 2-oscillator voice (going through the same filter and VCA) or to just use the ladder filter in the Mother 32 with an external oscillator, then you'll need something else to mix it with a second voice. Cascading VCAs are always a good idea :)
If you're not sure what modules you'll really need to make patches you're happy with, you should consider spending some time with a program like VCV Rack or another software modular environment to try and make a few basic patches or view other patches to get a basic idea of what you might need. Looking to non-modular hardware synths that you like for guidance can also be a good idea to get a feel for what most would expect to find in a typical synth voice.