Not as rare as getting a card-item in Ragnarok Online (with most bosses you can expect to farm a single boss for a full year, at least five times a day, and never see its card). XD But yeah, that happening in any one particular encounter is 0.01% (1% Bagon encounter rate times 1% Salamence SOS rate, if my math is right), so all I can say is good luck... though I wouldn't know why one would bother.
Well, it would if you consider the fact that at such a low level you're only looking at one or two points above everything else, so it can still die quickly to anything that hits it neutrally. It's one reason, I think, why the level 5 meta was developed (granted it may not be a thing anymore), so that all Pokémon can be used with a relatively equal chance of winning.
Exactly. Could you imagine a level 4 Bagon sending an SOS and all of a sudden a level 55 Salamence shows up? Couldn't run from it, couldn't KO it (unless you had a FEAR-Rattata, but then you'd be simply blowing through Focus Sashes); it would be a Nuzlocke killer. XD
True...but SOS battles would be super broken if that happened (which is why there's a general level cap)
