I know everything from Arrowverse but I am not following Supergirl. Only checked it's leaked pilot. Do I need to see the whole Supergirl till the crossover or is the basic knowledge of Supergirl + pilot episode enough to understand the crossover episode?
Yes. The episode doesn't go into much lore, and the basic premisses are explained to the Audience via explanations to The Flash/Barry.
The biggest issues would be:
How/Why Livewire became Livewire, why Silver Banshee hates Kara, and the bulk of the Kara/Jimmy UST.
But Callista Flockhart's character sums those up nicely (well, condescendingly) as well.