It is nearly impossible to write a Ranma story without someone thinking it is out of character or really any story in which you are not the original creator (and even then the reader may see it as OOC). You may try but it'll be your interpretation of the characters which everyone might not agree on. For some stories like Batman that has had multiple writers, multiple incarnation, and even rewritten parts of the story or changed the characters (early batman comics had Batman actually kill some of his opponents for example) it's difficult to say what is OOC or not because you don't know what version the person reading/writing will have as their source material (70's tv batman, 90's Batman, early batman, movie Batman, etc.)
Ranma is easier in someways in that it had one writer for all 38 volumes. Of course memory is fallible and people will forget parts. Not every reader of the fic or even every writer of fan fiction will have read the manga. Instead they may have seen the anime which was written by different people with their own views of Ranma. I wrote a few posts on the differences earlier; differences specific to the character Ranma and general differences. Even if the reader and the writer are using the manga they may have read different translations (online scan, VIZ, Russian version, etc.), which are different from one another. People who have seen the anime and read the manga may attempt to mesh the two together. Beyond that people will likely have read other fan fiction which will have also influenced them on the characters. Fanon is very rampant, in a earlier posting I listed many things seen in Ranma fanfiction that are countered by the manga; see here. That list is far from being a complete list I could add a lot more.
Then there are things like matter of perspective. From a first person perspective the manga is pretty dark and basically a tragedy. I have a post on that as well see here. From third person it's a comedy but not from first. So if you switch to a first person perspective the feeling of the story will change.
Then beyond that there are things like character decay (some characters become more 2 dimensional), PIS, and CIS. I went into some detail but no where near enough in this post here. My personal opinion on this for fic usage is that CIS (character induced stupidity) should be in full effect. Example of CIS, Ranma allowing himself to become distracted while fighting. PIS (Plot induced stupidity) and character decay though should be forgotten examples of these are Ranma's actions during the Gambling King Arc that was almost entirely PIS and directly counters the rest of the manga (none of the other arcs support it). An example of character decay is Kasumi becoming a mindless housewife even though in earlier arcs she would make comments, do things, and go on trips.
Beyond even that is that Ranma is set in Japan with a different culture than what most of the readers/writers of English fan fiction would be familiar with. The writer of the fic may have a better understanding of Japanese culture which would give them a different point of view than the reader does.
Basically the point of this post is, while I think it's good to attempt to keep the characters in character, no matter what you do someone will likely not see it as in character. So just write the story and don't worry too much about it being out of character. People will still read and like a story as long as it is good regardless of the characters staying in character. Of course if it's too out of character, you should probably just make it an original character.