Would a Selkirk Rex be calmer than a Devon Rex but sillier than a Persian?

Yes, that's a fair assessment. Selkirk Rex cats are generally more laid-back and mellow than the hyperactive, mischievous Devon Rex, but they still have a playful, clownish streak that makes them sillier than the typically sedate, regal Persian. They balance a calm lap-cat demeanor with occasional bursts of goofy energy, unlike the Devon's constant motion or the Persian's almost stoic stillness.

I've found that Selkirk Rex cats hit a sweet spot in the temperament spectrum. Their laid-back nature comes from that plush, teddy bear build - they're content to lounge around like furry throw pillows, unlike Devon Rexes that bounce off walls at 3 AM. But don't let the calm fool you, they'll still chase a crumpled receipt across the floor with a dopey expression that would make a Persian look like a grumpy statue.

You're describing them pretty accurately. I've lived with all three breeds and the Selkirk Rex is definitely a middle ground - they'll plop on your lap for hours like a lazy Persian but then suddenly decide to chase their own tail in a goofy zigzag pattern that no self-respecting Devon would attempt. Their silly moments come in short bursts though, unlike the Devon's constant mischief.

I've found Selkirk Rexes have this wonderful middle-of-the-road personality that's hard to beat. Their calmness comes from being content to snooze in a sunbeam for hours, unlike Devons that need constant entertainment, but their silly side shows when they suddenly do a belly-flop onto a toy mouse or stare at you with that squishy face after zooming across the room for exactly ten seconds before flopping again. Persians just look at you like you're interrupting their beauty sleep if you try to engage in such nonsense.

From my own experience, that's spot on. My Selkirk Rex will lounge like a lump on the couch for hours, totally chill compared to my friend's Devon who's always bouncing off walls, but then she'll do something utterly ridiculous like try to catch her own reflection in a spoon or pounce on a dust bunny with deadly seriousness. Persians I've met just stare at you like you're beneath them, so there's way more goofy charm in a Selkirk.