It may not be obvious until you build the control surface, that to trigger the playing of a “303” sequence (or other sequence, or just play a test note) you need to hold down one of the SID buttons and then press the menu button to toggle playing on/off. So if you’re planning a full CS with only one SID ever, you might want to add a button for this feature.
The best solution is just build it assuming you will one day put in 3 slave SIDs, because once you have one going, you might feel the urge for more… ;D