clays
"Clays". I am reading that as informal hand or machine thrown clays. I use a 26" SXS some of the time in my recently acquired trap shooting addiction. If I get to them quickly, then they break. I manage 22/25 - nothing better than that yet. I have FC tubes in both barrels but shoot the right barrel more often than the left.
What I have found is that the shorter barrels of my gun are "whippier" and thus less smooth than any of the longer barreled guns that I have been able to try. The 20" gun would/will be substantially more "whippy" than mine.
Shooting informally, I suspect that the chokes that come with the gun will work OK. If you find that you are shooting at longer distances, you will want to tighten the chokes a bit.
the short barrel and choke made the pattern expand to quickly to be used for shooting clays. Is this true?
Not exactly, no. Theoretically, patterns from a particular choke expand at the same rate regardless of other factors such as barrel length and gauge. In practice, you should pattern your shotgun so as to see what kind of patterns it produces at the distances that you are most likely to shoot. A pattern from your 20" barrel IC should not be very different than a pattern from my 26" IC. But is that the best choke choice for what we will be shooting? An IC choke might open too fast for formal trap shooting but be OK for Skeet (as I understand things).
Pete