Can you jump out, in combat, under 20 hexes from a planet? (Not the automatic jump when you don't move for two turns, but trying to escape, say...)

I don't think you can. If you can't, then hanging out next to a planet seems like a risky proposition unless you have spindizzy drives charged...

Just trying to understand combat a bit better before being thrust into it

Nope, Rip Drives will not engage under 20 hexes from a planet.