Ordered FM + Keys; totally new - a few questions

I will be receiving the Korg Volca Fm and the Keys this week and I'm psyched to dive in, start learning, and create music.

I have a few questions for the many knowledgeable people here:

• Speakers:
  • I'm not used to making music on a laptop, and want to have a "physical" setup for the time being with actual speakers.
  • I ordered the Presonus Eris 3.5 speakers.
  • Anyone have experience with these speakers, are they good for a starting setup?
• Grouping:
  • How does it work to "tie" the FM and Keys together
  • I assume I can use both at the same time running through one set of speakers?
  • What cables do I need and how to connect them to the speakers?
• Extension:
  • I saw some pretty stands for 4 volcas and thought that would be neat aesthetically (and give more of a palette to work with). What would be good options to extend the collection?
  • I see some reasonable deals for the Bass which I think might make sense to cover the low end?
  • Or would you recommend adding non-Volca options like the Roland AIRA T8?
  • Once you get to 4 units, can you still "tie it all together", or do you need a mixer to work more easily?
• Mixer and Modular: I don't see as much mention of the Volca mixer and modular, any experiences?