The following reflects my experience with Shimano shifters/derailleurs and 'standard' cable routing. I've never tried a travel agent, 'alternate' Shimano cable routing, or shimagnolo combinations.
In situations where shifters have more 'clicks' than cassettes have gears, it is assumed that the mechanic will lock out the shifter's final click with the RD limit screws.
* 10 speed shifters + 7/8/9/10 speed (except 740x Dura Ace) RD = 10s shifting on Shimano 10s cassette.
* 10 speed shifters + 6/7/8 speed DuraAce 740x RD (I used a 7402) = 9s shifting on Shimano 9s cassette.
* 9 speed shifters + 6/7/8 speed DuraAce 740x RD (I used a 7402) = 8s shifting on Shimano 8s cassette.
*7/8 speed shifters + 7/8/9/10 speed (except 740x Dura Ace) RD = 7s shifting on Shimano 7s cassette, and 8s shifting on Shimano 8s cassette (spacing between gears on cassette is the same for 7 and 8 speed, but 8s cassettes/freehub bodies are wider)
It is my understanding that 7900 FD/left hand shifter cable pull is different from all others, meaning you cannot mix and match. Moreover, 10s FD cage spacing will be narrower than 9s cage spacing, making use of a 7900 front mech with an otherwise 9s setup difficult, to say the least.