For a fighter's guild quest I have to kill Rufinus Alleius but if i kill him I wouldn't be able to do lord's mail.Should I kill Rufinus Alleius or tell Percius Mercius meaning I won't allow to get any quests from Lorbumol and plus its the last quest. If I don't do this quest will I be able to rank up still?

Well, it all depends on what type of character you're roleplaying. Are you really noble, or do you follow orders blindly?
And are you planning on doing the "Lord's Mail" Imperial Legion quest also?

You can still get the Lord's Mail even if you're not in the Legion.
You should be able to fully rank-up even by not doing the quest.

In the end, the choice is yours. That's why Morrowind is so great.

... And talk to Percius. He probably doesn't think it's one of the swifter ideas. Percius and Hrundi (Sadrith Mora FG) have enough quests to advance you in the Fighters Guild until...