I actually used a Lucario for my play-through of Pokemon X. I've never used a Heracross but if you look at there stats both of them can be used a physical attackers, Lucario can also be used as a special attacker if needed. Heracross has horrible special attack however. They're both fighting types, and capable of Mega evolution. If your using Charizard Y or are just using him as a special attacker in general (mine was Zard X a physical attacker) having a physical attacker on your team will be good for balance. If you have ZardX you could use a special, or a mixed Lucario. Heracross has more weaknesses than Lucario, including a 4X weakness to flying. 2X for fire, fairy, and psychic, Lucario is only 2X weak to fire, fighting, and ground, but is not weak to fairy. If your Greninja is the one you got from your professor then its not HA and has torrent as an ability, meaning it can't use protean to get rid of its dark typing, leaving it weak to fairies. Having another Pokemon on your team weak to fairy (Heracross) might not be fun, but on the contrary Lucario and Snorlax are weak to fighting and Heracross is not. As far as coverage goes, Lucario has a basic fighting and steal movepool but doesn't get a lot of good attacking steel type moves via level up so you would need TM's or to breed egg moves like bullet punch. Heracross learns mostly Fighting and Bug type moves by level up. Lucario does get one great move that is not a fighting type move, Dragon Pulse. My in game Lucario had this move and it was good for type coverage because my team really needed something for dragons, without using an actual dragon type. At the time I didn't use fairies and if you aren't up for fairies here's a dragon type counter. If you have your heart set on Heracross though, you can always use ice beam on your Greninja. In my opinion Lucario is the better choice just because it has more ways to be used, based on how your team is built, but I'm also personally biased because I love Lucario with my whole being <3
As far as Noivern and Tyrantrum go, here are your options...
Noivern is a special attacker, Tyrantrum is a Physical attacker. Once again, look at your team and decide what coverage you need. Since Charizard can be physical if your using the zardX mega or special if its zardY or not mega at all and Greninja tends to be special too you might want a physical attacker. Both of these Pokemon come at a big price weakness wise though. Noivern is X4 weak to ice, I don't know if that alarms you as much as it does me, but I really dislike using Pokemon with 4X weakness to something. It is also weak to fairy, dragon, and rock by 2X. Noivern Might blend pretty well with Lucario since Lucario isn’t weak to fairy, and can be a good switch in if you predict a fairy type attack, and Noivern can switch in for Lucario if your expecting a Ground type attack (Charizard can also switch in if its not ZardX because it’s immune to ground and resists fire). Noivern shares fairy weakness, with Heracross, which could be covered by Lucario.
Tyrantrum is 2X weak to ice, fairy, fighting, steel, dragon and ground. It shares two weaknesses with Lucario which makes them not so good of a pair, but only shares fairy weakness with Heracross, so they could work together weakness wise, but not quiet as well as Lucario and Noivern do in my opinion.
Tyrantrum also has terrible speed, and special defense. A STAB ice beam could easily take it out, same with Noivern, Charizard will cover ice weakness but many Pokemon who use ice moves can also learn water moves which is not fun for charizard. Your only hope would really be to use zardY drought and solar beam which has become a very popular pairing for zard Y movesets. Almost every zardY carries it. But if your using ZardX theres really not much you can do for water types other than switch out to Greninja to resist it.
As far as the gyms and elite four go there is the bug gym, the rock gym, fighting, grass, electric, fairy, psychic, and ice. The elite four uses steel, fire, water, and dragon type Pokemon.
Heracross would have fun cleaning up the psychic types, but wouldn’t have so much fun in the fairy gym where lucario could handle them both okay. Lucario wouldn’t like the fighting gym though. The elite four’s fire types wont be fun for either of them, but lucario, if it uses dragon pulse, could take care of the dragon types of the elite four without putting Noivern or Tyrantrum at risk.
Tyrantrum wouldn’t have fun with the ice, fighting or fairy gyms, and wouldn’t enjoy being thrown around by the dragons of the elite four even though it has an advantage its paper thin special defense just wont hold up long enough. Noivern wont like the dragons either but it at least has some better special defense then Tyrantum. Yes Tyrantrum has lovely defense, but its just not quiet as balanced with Noivern having base 80 for both defenses, and Tyrantrum having base 119 and 59.
Overall, Noivern has a better base stat total than Tyrantrum, and Lucario has a better base stat total then Heracross. Noivern and Lucario can actually work together nicely where Herachross and Tyrantrum don’t seem to complement each other the same way as Lucario and Noivern do each other.
In my opinion Lucario and Noivern would be better but don’t just trust my opinion. Get opinions from a lot of different people, use which ever Pokemon YOU like best. I hope I helped. Sorry I didn’t take megas for Lucario and Heracross into consideration, if you wanted more in depth info on the megas just ask.
(Also this is the first time I’ve answered this type of question so I’m sorry if it sucks.)