Unfortunately, it's a little more complicated than that (sometimes).
We'll take the SEX as a partial example -
The 0.1uF coupling caps really should stay at 0.1uF, with voltage being at least 400V.
The 1.5uF parafeed caps could be 0.75uF to 5uF if you wanted, with at least a 600V rating.
In the Quickie, the high voltage supply is 36V, so restricting your shopping to 630V caps will make your job tough. (The stock caps are 250V rated) Additionally, the impedance loading the output of the Quickie may let you get away with less capacitance.