The problem that you might hit is that the 9s ring spacing is a little different to 10s - however, there are some workarounds and you may have to accept one compromise to how the system "should" function.
If you run a 10s crankset with a 9s chain, especially the Compact (50/34 version) if you use the inner ring and the smallest one or sometimes 2 sprockets (depends on the rear triangle length of the frame), the rivets on the chain can catch on the outer ring's lift ramps / pins which gives an effect like chain suck. Even 10s chains used to do it occasionally, on really short rear triangles.
The workaround is to use a Campag 10s chain (Veloce is still widely available, Record from
Condor, only, now) which minimises the issue but potentially then gives you a problem with the front mech, because the cage is too wide for good chain control going from the big ring to the small, if you are heavily cross-chained in the other direction.
Note, before people dive in - it's not all about the limit screw - even with impeccably-set limit screws, it happens. It's the same on Shimano.
Depending on the FD version you have, there are a couple of additional workarounds.
If you have the type with a perforated front to the cage (the 9/10s version) there is a plastic insert that you can put in (it is present already in the 10s version, can be added to later 9s versions, FD-RE016), which effectively narrows the FD cage at the critical spot. In that case, you can set the FD up normally and you will be able to get all 9 sprockets at the back without having to trim, when on the large chainring.
If you don't, there's a different FD set-up routine which can help slow the FD cage down as it finishes it's stroke towards the low limit screw, meaning that the chain hits it and bounces back onto the ring, if it's heading for unshipping to the inside - this set up routine is a bit of a faff and sometimes means that you need to do a Shimano-esque single click downshift to trim the FD on big-to-big to avoid chain-scrape - but it does work. Essentially you need to tension the FD cable with a frame adjuster or an in-line adjuster so that as the inner ring limit surface FD gets close to hitting the inner limit screw, the spring on the FD cage has to "fight" some tension on the gear cable - think of it like a "soft-close" drawer. It's essentially the same as is now used in a correct set-up on both Campag and Shimano mechanical 12s FDs.
We went through all of this years ago on the teams, when we ran some hybrid systems especially on the smaller and / or domestic teams, rather than what would now be WT/ GT, for a variety of reasons ... even on GT teams, because various non-standard set ups were used, there was some "finangling" required ...
