As of the C64, they are all sloping frames now. Sloping frames became a thing for Colnago around 2005 or so (don’t quote me on the year) but you could still get all sizes in traditional sizes in 1cm increments as well. The sloping models were produced in even sizes only (52, 54, 56, etc). The sloping sizes’ geometry would correspond, fit and handling wise, to the traditional frame 4cm “bigger”. For instance if you wanted a 56s (sloping), that would correspond to a 60 Traditional (horizontal top tube). Then with the C59 they made it so that the traditional geometry was only available in the odd numbered sizes and the sloping only available in even numbered sizes. So, in my case for example, a 56s corresponds to a 60 Traditional, which would likely be perfect for me, but since I wanted a horizontal top tube, I got the 61 for my C59. For my C60 I have a 59 Traditional. In a C64 I would get a 56s. And so on.
As for custom geometry, yes it was always available through special order but I don’t know if that is an option with the C64. Oh, and by the way, to answer your other question... no, you can’t get the C64 in traditional geo. Part of the whole C64 thing, if you believe the marketing, is that the increase in vertical compliance is largely due to the sloping top tube and the resulting extra seatpost length allows more leeway for “tuning” in some compliance.
Custom geometry is/was certainly an aspect that separates Colnago from most, if not all, of the mainstream manufacturers. And it is the lugged design that allows this. I hear people talk about how the lugs are what gives Colnago it’s magical ride quality blah blah blah. Bollocks. I believe the fine tuning of a carbon frame can be better done with well designed and sometimes complicated layup schedules within a mold. That said, the lugs allow for very fine tuning of the geometry (which a few common molds do not) for each and every stock size as well. And this is no small thing when it comes to finding that Goldilocks like frame that both seems to “fit” as well as handle, well, like it was made just for you. Just look at the individual geometry numbers for each component of each size. Head tube angles, seat tube angles, top tube lengths, chainstay lengths, etc. You don’t see much in the way of the same chainstay length for example between sizes. Sure, making all the chainstays the same length makes it easy for manufacturers to use common molds between several, or even all, of their frames, but that doesn’t mean it’s best for the rider. Take the top tube lengths of the 3 sizes that I could, for example, work with for my personal setup. The top tube lengths are all within a 9mm window, from 577 to 580 to 586 in the case of the C60 for example (sizes 59T, 56s, 61T). Those are pretty small increments when compared to some manufacturers which might have a whopping 15mm or more between top tube lengths of two adjacent sizes. At the other extreme, when Giant first introduced the “compact” road frame to the peloton, there were initially 3 sizes... S/M/L... with fine tuning for fit done with various length and shapes of seatposts and stems. That didn’t work, at all, as frames from a handling perspective were often simply too big or too small even though one might be able get the 3 fit coordinates (saddle, pedals, bars/stem) just right.
So, that’s kind of the long answer to your question, but I’m pretty passionate when it comes to bicycle geometry from both the fit and handling aspects, as well as how it ultimately builds up into the finished bike, so from the aesthetic perspective as well.
But I still don’t know for sure if you are able to order a custom geometry in a C64. I would think “maybe”, but you’d have to check for certain on that. Actually that’s a good question and I will check myself at some point soon. The current size range, however, is still pretty extensive so I would expect the custom geometries to be pretty rare these days. What I do know is that a 56s would be my size, without question, in a C64. Versus the old days, when it was a laborious exercise in deciding which of maybe 4 sizes (all of which would work), you should choose.