When would you change "hybrid transaxle fluid"...What symptoms would you have?

yes, that would be the same fluid, the tranny is described in many different ways, but toyota calls it an ecvt. the idea is to change it before the debris destroys the transaxle. some do it as early as 5,000 miles, then every 30k or so.
but yes, now is as good a time as any. no guarantee it will fix your problem, but it might help prevent future ones.

 

Yes, good idea to change it. I would strongly recommend to use Toyota ATF WS fluid, and nothing but. Second gen has different bolt sizes/types, for the drain and fill bolts. You should also change the aluminum washers on those bolts, just for security. Check with dealership parts department, they should have both the fluid and the washers.

You should be ok with 4 quarts of fluid, I believe it'll take about 3.5? The correct level on refill is achieved by filling with the car level, just keep filling till it starts coming back out.

I don't know the 2nd gen drain/fill torque spec. FWIW, 3rd gen is 29 ft/lb for both. On third gen the drain and fill bolts are identical. Also, I would research, make sure you've found the correct two bolts, there is also a coolant drain bolt I believe? Also, it's good practice to remove the fill bolt first, just in case you encounter problems breaking the bolts loose; you don't want to drain the fluid, then find the fill bolt stuck.

And when filling, the simplest and most effective method (at least what I've found) is to just run a hose extension on a funnel down from above. Best to get a helper to pour and watch below. Once you're over 3 quarts added, just add around 100 cc at a time, till the overflow starts, and you're done.

Maybe someone will weigh in with second gen drain/fill bolt locations, torque specs.

 

The "skip" you're feeling probably has nothing to do with the transaxle. As mentioned, it is continuously variable and there's no hard "shift" as you'd see in a regular automagic transmission. There could be something electrical or electronic with the hybrid drive causing that feeling, like the electric motor getting power from the battery.

So changing the fluid is a good idea, but shouldn't change the way it feels.

The capacity is 3.9 qts.

 

The Gen2 Prii DO NOT have any form of "transmission" as it is normally thought of. There are simply two motor-generators connected to a planetary set that drives the wheels. The DIFFERENCE between the speeds of the two motor-generators determines not only the direction but speed of the drive. There is no belt, no clutches and no gears to change.

It is VERY important to use WS or WS equivalent fluids due to the HIGH VOLTAGE wiring in the "transmission".

 

I change the Transaxle (combined transmission and differential) with ATF WS at 30,000 miles, then 90,000 miles, then every 90,000 miles.

 

Like Andrew said the skip your feeling is a minor misfire most likely from worn spark plugs.
Not big enough to throw a check engine light ......yet......
If the car has been good to you I would also at the same time as the plugs replace the spark coils too that are mounted on eAch plug.
There called cops coil on plug. You can buy them Toyota oem online for $75 each. Don't bother with cheap after market cops they suck. New plugs and cops good to go.

 

Would change it ASAP if this is the first time.

Symptoms include a feeling of financial loss (around 4k at the dealer), and may also include an uncertain feeling while driving.

Lucky for me the prior owner was the one who experienced the 'transmission' failure, and he had no clue until it actually happened far away from home. Yes it was covered by the California warranty, but there is little doubt that any part of his experience would've been as worthwhile as simply draining and refilling the ATF.