To add to the 100 / 120mm fork side of things. I’ve had an Io (bought the frame only) for a couple of years now. Best measurement is axle – crown, as my original MX Pros at 100mm were 15-20mm longer than the rebas that are now on it. I changed the Rebas to 115mm last night, and will be off on a night ride this evening with them. The axle – crown is the same as the original MX Pros, so it should ride very similarly.
With the Mx Pros, it’s very happy downhill, feels like you can ride over most things, and very chuckable. with the 100mm rebas, it feels a lot more racy and twitchy, but enormous fun.
As said above, if the Alpine is anything like the Io’s geometry, it should be a very fun bike. I’m riding the 16″ and am 5’7″ tall, if that helps with sizing.