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This might work, Do you know the current/voltage specs of the lamp ?

Both F21T5, as well as F28T5HE are 0.21A (for 50Hz) lamps, so running them on a F20T8 ballast would overdrive them.
I guess the flicker came from overloaded electrodes, where they start to rectify, what in case of the magnetic ballast cause it's saturation, so quite huge current spikes. The correct ballast would be the PLC-18W (18W "quad-tube" CFL, rated 0.22A)

And beside that, the arc voltage of both lamps is quite high (125V for F21, 170V for F28) to run on a simple inductor ballast on 230V mains.
What could work is the "PLC-18W" (DULUX-D or -T 18W) ballast in series with 2.2uF/400VAC (1000VDC) capacitor. This will form a ballast with quite a stable operating current (controlled by the choke saturation) more than 400V available for arc reignition, so it would be stable even with nearly 180V lamp...

I think yes... Or connect 2 4-5uF's in series (preferrably with voltage divider resistors to prevent "interesting" voltages apearing on the midpoint between capacitors)

But since the current is 0.21A and voltage is 125v - 170v shouldn't the extra current from the T8 ballast compensate for the power since Power = Voltage x Current?

The 125V is the F21T5, 175V is the F28T5.
The voltage is always dictated by the lamp, so the F21T5 would have about the same 125V regardless if the current would be 0.15A (from PLS 5..11W ballast) or 0.37A (from the F36T8 ballast). What would vary is the real power delivered to the lamp: The 0.15A ballast would deliver there 125*0.15*0.9=17W, the 0.37A ballast would deliver to the same lamp about 42W (the 0.9 is the power factor covering the shape mismatch between the nearly rectangular voltage vs nearly sinusoidal current)

So when you want the lamp to operate at rated power, you need to supply it from a ballast matching the rated current. And for the 0.21A the closest one I have found is the 4-tube 18W CFL (PL-C 18W, DULUX-T or -T 18W,...)


The fan motor capacitor should be OK, the motors expose the capacitor to about 400..450V and in fans the capacitor is connected all the time, so it would be rated to withstand that for long enough time.

Only a note: For the 175V F28T5 you may have troubles with starters - the 175V could cause the starters falsely close. The solution would be the electronic starter (PulseStarter, S10e,...), where the "Fault mode" timer make sure the starter stop interfering with the tube after the start (these starters preheat for about 2.5 second, then about 0.5seconds for ignition attempts and then they switch OFF till they get reset by disconnecting the power, so switching the light OFF)