I've put a 10sp cassette wheel onto my 126 spaced steel frame. I use downtube friction shifters and a Campy Nuovo Record rear derailleur. It shifts flawlessly. The cassette is Shimano Ultegra, a 10sp Shimano chain, 10sp Chorus crank. I can take the wheel off and on my frame without any problem. I just spread it slightly so the wheel slips in. What impresses me the most is the silence from my drivetrain.

You really don't need indexed shifting. With this setup you need a light touch to get the shift to happen. The Hyperglide chain and cassette work together to make the shift smooth and silent with the ramps on the cogs. You are rarely between gears as the spacing is so narrow. You don't get that clunk that you get with old 6 speed, nor do you get the click from the brifters.

Way back when indexed shifting came out the thought was the general public needed a reliable shifting mechanism that didn't require constant adjustment. This was to sell more bikes of course. Over time, Shimano and Campy improved their systems to where we are today. Indexing was always part of that system, but ramps on the cogs did more to improve shifting than are given credit, and make the indexing besides the point. I'm not saying that I don't like indexing, but it really isn't all that necessary with 9 or 10 speed.