Always shut the amp off and wait 30 seconds to a minute before you replace any tube in any tube amp. During the wait period the high voltage will bleed off and you won't get a transient spike when you pull the tube. Of course use a cloth or something to pull the tube so you don't burn your fingers.
The 6829, 5695 and 12AV7 are plug compatible with the 12AT7 and will work in the amp in the center position. They are also somewhat lower in gain than a 12AT7. How they will sound in comparison to a 12AT7 is anybody's guess? I have never used any of these three tubes in my own ST-120. The VTA board on the ST-120 has been optimized for 12AT7 tubes.
My personal favorite tube on the ST-120's center position is the Mullard CV4024 which is a 12AT7 equivalent tube.
As an aside > Substituting LOWER gain tubes into a circuit usually does no harm BUT how good it will work is questionable. From personal experience I would never substitute a HIGHER gain tube in a circuit. IOW never substitute a 12AX7 tube (mu = 100) for a 12AT7 (mu = 60) Higher gain tubes can sometimes cause an amp to oscillate, squeal, motorboat or become unstable especially at higher volume levels.