i have been wondering this for quite some time now. I have 3 Universal Therm-O matic rapid start 2lamp ballasts that run 50w mercury lamps quite nicely. However, they are too large to install in a fixture, so I am wondering if 4 pl13 chockes in series or parralel would run the lamp. It would equal 52w, which should be enough to run a 50w  , but I am not sure. Would it work?

Its a 90..95V lamp, thats fairly high arc voltage for 120V mains. Probably too high to be able to run with chokes

You might be able to use different wattage autotransformer ballast and the chokes to get its output down though

Nope.
With ballasts you can not just sum the power, the ballasts do not work that way.

The ballast is a source with some open circuit voltage (OCV) and some output impedance (and in order to be not that simple, the impedance is very often not linear, so changes with the load), that when loaded with the rated lamp transfer the rated power to the lamp. So it is the combination of all: the open circuit voltage (ballast property), the ballast output impedance (ballast property; and has two components - resistive and reactive) and the lamp arc voltage (the discharge lamps tend to force certain voltage across their input terminals), what results into certain power transferred from ballast to the lamp.

To be even more complicated, the ballast should be able to provide condition for the lamp startup (arc ignition, plus electrode and arctube warmup), in real life it means some minimum OCV - rule of thumb is at least 2x the maximum arc voltage, plus eventually ignition spikes (extra voltage generated separately out of the OCV-impedance equation; usually an inductive kick or ignitor pulses)

So if you want to use certain ballast to feed certain lamp, you have to make sure the combination of the OCV, ballast impedance and the lamp arc voltage yields the power transfer you need.

With the F40T12 ballast you get an OCV of at least 220V, feeding 40W into a 105V lamp. The MV is a 95V arc lamp rated to be fed by 0.6A so due to the reactive ballast impedance the current would remain 0.43A, what means about 37W into the 95V MV. Underdriven, but may work. The OCV requirement of the MV is 200V, so any F40 ballast would fulfill that requirement.

I guess by the "PL13W" you mean the US market PL-S13W U-tube with an integrated starter.
The "13W"on the PL-S13W ballast means the ballast has such OCV and impedance, it delivers 13W into the arc voltage of the PL-S13W lamp.

The PL-S13W is a 65V arc lamp, so suffices with the mains 120V as the ballast OCV, so the ballast may be just a series choke.
That means the OCV of any number of these chokes in parallel would be still just the 120V mains. That means the lamp most likely even won't ignite.
And if it ignites, when the arc voltage exceeds about 65..70V during warmup, the arc will most likely extinguish (the "2x arc voltage" rule would not be fulfilled).
And for the warmup, the 50W MV electrodes would be quite overdriven - as each PL-S13W ballast delivers about 0.3A into low voltage load, it would be about 1.2A total, the MV50W expect no more than 0.9A during the low voltage state.