If a card that's already been on the battlefield for a full turn becomes a copy of something else, it'll still be unaffected by summoning sickness
Though exiling four cards a turn sounds like a lot of your deck gone, lets do the math: 100 cards, minus the commander, minus your opening seven-card hand, minus four draw steps by the time you cast this, leaves 88 cards in your library
This version of Kellan lends itself to a bit of a Voltron build thanks to its double strike and ability to buff your other creatures based on how equipped it is
It's a slow card and will be a magnet for removal if you're exiling stuff your opponents want back, but in a dedicated +1/+1 Counter deck it's a solid way to slow down opponents, plus you can even exile your own stuff to re-use ETB triggers or protect from a future board wipe
Once a spell has been copied on the stack, its treated as any other spell and subject to the same weaknesses
Is there a cool combo between token producers and populate that you want to share