You'd get more help if you supplied more of the story yourself, but just 
so you don't go away thinking this list is useless, here are a few ideas.

ryuuji grey wrote:

Im planning on wrighting an azumanga daiou ranma crossover,
  but i only have a vauge idea, of where to go with it, i guess i should start by saying it will be based on the anime, as i do not have the manga, 
  

You probably should have told us here which series you have only the 
anime.  Both?  Ranma?  AZD?  There really isn't that much difference 
between the manga and anime for AZD, not enough for it to be meaningful 
to most fanfics.  But I somehow thought you were probably saying above 
that you only have the AZD anime; on third reading, this is unlikely to 
be what you meant.

   
  the big problems are the fallowing, 1. azumanga is relivively peacful, Ranma is not.

The Ranma series isn't.  Quite a bit of Ranma fanfic is.  See Joseph 
Palmer's stories for examples.

 2.the way of crossing over ranma characters to azumanga characters
  

Any way you want to.  Just make your big assumption evident up front, 
and your readers will swallow it more easily.  One standard way would be 
opening with something like the following: 

Tomo sorted through the day's mail as she kicked her shoes off.  "I'm 
home!" she called as she picked out a postcard addressed to her father.  
"Bringing Ranma from China," it said, and it was signed "Saotome."  Tomo 
shrugged; not her business, she figured.

Another, less common, way would be:

Ranma waited outside the classroom to be announced.  Involuntarily, she 
shivered.  Starting over again... the reasons tore at her heart:  
Akane's tragic death, the lock on her curse, her father and mother 
disinheriting her.  Only the thought of what Akane would say to her kept 
her from committing suicide.

Ranma heard Miss Yukari's cheery "Come on in!".   Bucking up her 
shoulders, she slid the door open and walked in to face the unknown.

   
  to get an idea of the whole picture though i guess i have to start with the little things
  1. ranma meets or does not meet the fiancees
  2. ranma becomes a kid before or after meeting the fiancees, when
  

Becomes a kid?  Huh?  It sounds like you do have some sort of fixed idea 
about your story, but you never told us what it is.

  3. ranma is or is not in love with one or more of the fiancees
  4. Saotomes financial status (the summer home plays a big role in azumanga, if the saotomes cant finance it there has to be a replacement
  

Huh again?  Why do the Saotomes have to have a summer home?

  5. the trigger, there has to be a reason for ranma to become a girl as a public appearance and learn well enough to get into highschool
  6.ranmas strength. chiyo chan is very weak and unalthletic, ranma is extremely athletic, although ranma would be weaker she could easily take down most opponents arguably and depending on the time ranma makes the change, how should ranma handle his strength, show it off? hide it? 
  

Ah!  Now I get it!  You don't want to write a crossover at all; you want 
to write a fusion:  AZD with a 10-year-old Ranma-chan replacing Chiyo-chan.

My first thought is to wonder why on Earth you'd want to.  Chiyo-chan is 
a very unusual character, and a good source for a lot of laughs.  Why 
get rid of her?  Without Chiyo-chan, in my opinion you wouldn't have 
enough of AZD left to make your proposed fusion worthwhile.

If you have some story idea featuring a ten-year-old Ranma-chan in an 
AZD setting, then why not just add Ranma to the AZD cast?  Why do you 
have to replace any of the characters?  If, for some reason, you DO have 
to replace one of the AZD characters, then Sakaki is the obvious 
choice:  extremely athletic, problem with cats.

  7.attire, ranma wouldnt even wear the boys uniform in school, is there something that could get him into female attire?
  8.Fiancees appear in azumanga? should they appear? why and how? (note regardless of this Ryouga will make an appearance. he gets lost to often not too.)
  9.lock of the curse. should ranma be locked
   
  yeah, its alot to ask for ideas and solutions to practicaly 10 problems, even if an answer to one might solve another. but i hope you guys can help me out. alot of people stop reading fanfiction because of plotholes or pathetic reasoning (i know I do.)
  for events, I want the chain of events to be belivable.

  

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