On my first playthrough I did a minimalist play through doing only the main quests with a few side quests. I was an energy weapons character and I used a Fully modded Laser Rifle (sans the scope) as my weapon of choice, I sided with ncr.

I'm thinking for my next playthrough I will run another "good karma" energy weapons character and side with ncr, but do as many side quests as possible. (is it true that if I active "Beware the Wrath of Ceasar" and "Dont Tread on The Bear" with Mr.House then I can do as many Legion quests, NCR quests and Yes Man quests as I want without losing loyalty to NCR or Legion?)

Then I want to do a melee weapons and guns character have "bad karma" and side with legion.

What are your ideas?

If you side with one (Meaning Legion or NCR), you cannot side with the other.

If you work with NCR for an amount of time, the Legion will shun you, and will send assassins after you.
If you work with the Legion for an amount of time, The NCR notes you as a terrorist and sends bounty hunters after you.

As for House/Yes Man, you fail "Don't Tread on the Bear" (that means you cannot help the NCR) after you finish up all the side faction quests, and move on to the substation quest.

An example: If you side with the Legion and make it all the way to the final battle, and decide you don't like them, your only option is to activate Yes Man and go the independent route.

I recommend Legion to anyone who hasn't done their quests, because they are actually quite interesting and bring a different flavor to New Vegas.

Overall, however, I'd probably recommend Mr. House if you want the best outcome for Vegas and the mojave, and you like being in the good karma range. Some people have different opinions on this, however, so you will have to make your own choice in the end.

EDIT: It is more than acceptable to support the NCR if you are more of a good karma person as well, because I think NCR is the only way you can get a complete good ending.

I mean I heard if you do too many missions for Mr.House or Yes Man and Beware the Wrath of Caeser and Don't Tread on The Bear are activated, but not failed, then you can do missions for Legion and NCR without worrying about being declared a terrorist/enemy.

There are some quests that can not be completed for one faction without failing it for another.

As far as I know, this is not true.

When you "fail" Don't tread on the bear or Beware the wrath of caesar, it means any chance of doing the main quest line for either is screwed.