so, for your lucario even thought it gets STAB on flash cannon i don't really see much use for that move. sure, its helpful to take out ice and rock types, but vacuum wave or aura sphere can do the job on just about every rock type and ice type. since your worried about jellicent countering lucario i suggest you use shadow ball over flash cannon because it will be super effective on jellicent and after a nasty plot boost it can possibly OHKO jellicent ( depending on spec def EV's).
to counter steel types just use volacrona or lucario. if you use lucario be careful of magnet pull magnezone because those almost always carry HP fire.
hope i helped a little bit =)