Can one character be a member of the dark brotherhood, the imperials or what ever else with out too much overlap ?...seems like the thieves guild and maybe assassins would both have qualities that could overlap ...what do you think ?

You can join all factions.

The only thing you can't do is join both Stormcloaks and the Legion. Of obvious reasons. Other than that - yes, your character can do absolutely everything. And no, it's not very believable. That's why people create multiple characters

You can join everything you just have to make one choice for the Stormcloaks or Imperials in the civil war quest line.

It sounds to me like you're asking is it crazy from a roleplaying standpoint. If that's what you're asking I'd say yes. It makes no roleplaying sense for most characters to do everything.

No you can't. You have to choose between Stormcloaks and Imperial Legion - can't get both. And if you'll involve with DB you have to choose between destroying it or joining in it.

Uhm, yes, if you join a war you need to choose sides.

if you pick a side then you cant pick the other in the same game, effectivly locking you out of it for awhile, that is what he is saying

but joining too many faction is crazy but it is possible

lets do it all now and live with our regrets tomorrow

You can do it, you just have to be crafty with the role playing;

My Renegade Imperial: Crixus fled the Dominion after one of their murder squads wiped out everyone her every knew, needless to say he bore a deep hatred for them.

Upon surviving Helgen he figured his best bet was to get as far away from everything as possible and went to the college in winter hold. After Battling the Thalmor there and taking over he launched a counter attack on the Thalmor at their Northern Fort and freed some people as well. Using the Stormcloacks as a shield he wouldn't have to worry about the Thalmor bothering him. The Empire would leave him be because the College would put the civil war in the Stormcloak favor, and the Stormcloacks would leave him be because the college would put the civil war in the Empires favor.

Wanting to expand his power secretly he joined the theives guild in hopes of obtaining their loyality and their intel gathering sources for his "Secret Kingdom". With the Theives Guild he soon found all the additional funding he needed and all the intel he could ever want. And this also gave him power over economics in the Skyrim area. With the the Theives guild in his hands he expanded their operations to every corner of skyrim.

It was at this point he felt safe enough to publicly seek the Title of thane at the holds of Skyrim to have some legitimacy to his claim of power. While do this he also saw the strength of having the Bards college in his pocket for good press (propoganda) and so everyone would know of his greatness. Whiel at teh same time being able to control the knowledge of his Secret Kingdom.

It was during this time of seeking legitimacy that he learned he was Dragon born, after talking witht eh Grey berad he became convinced that it was a sign that he would rule all of the Empire one day just like Talos. but to do so he would need his own private army. So he turned to the Companions, earnt heir respect and obtained their loyality and highest leadeship postion. he then pocressed a Private Army like no other.

Shortly after gaining his Companion army he was contacted by the last of the Blades, hearing that the Thalmor could be responsible for the dragons he agree to infiltrate their embassy. He slaughter every Thalmor he could, some for the mission itself, but most for his own personal reasons. But it was after this attack that he learned that Stormcloacks "maybe" nothign more than the Thalmor puppets, but before he could think to much about the implications of such information he recieved word that the Thalmor had attacked the Theives guilld looking for an old Blade member.

After Helping the Blades and learning the truth about what was to come he attacked the Thalmor Embassy once again out of revenge for attacking the Thieves guild HQ, he spared no one outside the compound. Seeking information on how to kill the world ender he approached the GreyBeards again but they would not give him the information he sought because he woudln't let them know about the blades.

Not knowing what to do he sought out the Dark Brotherhood (they had to know how to kill a world ender right), but it was through here that chance and fate gave him opertunity to gain control of the Empire. He would kill the Emperor and then win the Skyrim Civil war for the Empire. Who else but the Dragonborn Hero of the Empire could take up the Thrown? They would practically beg him to lead the Empire, if not well there would be nothing they could do to stop him. Also since he woudl only join the legion after the death of the Emperor he wouldn't break his vow to be loyal to the Emperor (hey every man has a code).

After crushing the Stormcloaks he knew it would only be unstoppable he had:

A Mage force (Mages College)
Spies and Control of Economics (Theives guild)
Private Army (Companions)
Secret Police and Assassins for the cause (Dark brotherhood)
Propoganda and the Stories (Bards College)
Political Alliances (Being a Thane in all the holds)
Clout in the Empire (Civil war Hero)

Skyrim was his and no one could take it from him. He was touched by the gods (and pretty much on a first name basis with alot of them) themselves it was his destiny to Rule and no one would stand in his way. So began the First days of the New Empire............

So to answer your question, yes you can do all the quest from a role play perspective and justify it.