Firstly, non-vanilla amplifiers should be allowed. However, if you go higher than vanilla, usually you're best off using the max value, and using a random value between a range will usually be extreme. One problem is that not all potion effects work with a max amplifier, and I'm not sure if there's a way to identify which ones do.
https://github.com/HopperElec/RandomEventsPlugin/blob/41335a52553a39f7478948da82aa48b35dcb2d2f/src/main/java/uk/co/hopperelec/mc/randomevents/eventtypes/RandomEffectEvent.java#L39-L42
Firstly, non-vanilla amplifiers should be allowed. However, if you go higher than vanilla, usually you're best off using the max value, and using a random value between a range will usually be extreme. One problem is that not all potion effects work with a max amplifier, and I'm not sure if there's a way to identify which ones do.