Since not seeing all that many stories being mentioned in this thread... figured would dump my own list in. Slayers and Ranma crossovers is a old crossover combination, most have been abandoned for a long long time. And since it is such a old category, most predate the introduction of the FF.NET multi-crossover category system. Some even predate the introduction of the multi-chapter system, thus each chapter is its own link.
Some of the below are no longer available, but here is what I currently have that I know has more than a mere mention of Lina or Ranma in them...
RANMA-MULTI 'Eezoon' by Nabeshin Danbei Ayanami - [Dld Date=07/25/2011] - [Dld Size=249kb] - [Dld File Cnt=6] - [Dld Status=Inactive]
RANMA-MULTI 'Grand Tour' by DrunkenGrognard - [Dld Date=07/15/2009] - [Dld Size=657kb] - [Dld File Cnt=21] - [Dld Status=Complete]
- - [MD Source=FF.NET] - [MD Category=Ranma] - [MD Rating=T] - [MD Word Cnt=119993]
- - [MD Summary= A crossover fic. Ryoga's curse of 'lost' threatens the fabric of the multiverse itself. The USS Voyager is swept up in his wake of chaos. The TSAB steps in to clean up the mess recruiting Ranma to aid them but it may already be too late to catch him...]
- - [Direct URL] https://www.fanfiction.net/s/4869171/1/
- - [Author1 URL] https://www.fanfiction.net/u/1677919/
RANMA-MULTI 'Just won't die' by Jeffrey OneShot Wong - [Dld Date=05/31/2006] - [Dld Size=14776kb] - [Dld File Cnt=33] - [Dld Status=Inactive]
RANMA-MULTI 'Perfect Chaos' by Montymouse - [Dld Date=06/21/2007] - [Dld Size=406kb] - [Dld File Cnt=9] - [Dld Status=Inactive]
RANMA-MULTI 'Soul's Destiny, Blood's Inheritance' by kokatsu na tenshi - [Dld Date=11/02/2004] - [Dld Size=520kb] - [Dld File Cnt=17] - [Dld Status=Complete]
RANMA-SLYR 'Dragon's Bond' by Morden Night - [Dld Date=02/26/2004] - [Dld Size=829kb] - [Dld File Cnt=9] - [Dld Status=Inactive]
RANMA-SLYR 'Leaving, chapter 1 Chance encounters' by Kuraiko Kurohoshi - [Dld Date=04/19/2000] - [Dld Size=38kb] - [Dld Status=Inactive]
RANMA-SLYR 'Leaving, chapter 2 A new life' by Kuraiko Kurohoshi - [Dld Date=04/19/2000] - [Dld Size=48kb] - [Dld Status=Inactive]
RANMA-SLYR 'Leaving, chapter 3 The hunt for Ranma' by Kuraiko Kurohoshi - [Dld Date=04/19/2000] - [Dld Size=40kb] - [Dld Status=Inactive]
RANMA-SLYR 'Leaving, chapter 4 Open lines of communication' by Kuraiko Kurohoshi - [Dld Date=04/19/2000] - [Dld Size=58kb] - [Dld Status=Inactive]
RANMA-SLYR 'Leaving, chapter 5 Promblems, part 1 Mighty Past' by Kuraiko Kurohoshi - [Dld Date=08/08/2000] - [Dld Size=54kb] - [Dld Status=Inactive]
RANMA-SLYR 'Leaving, Prologue' by Kuraiko Kurohoshi - [Dld Date=04/18/2000] - [Dld Size=9kb] - [Dld Status=Inactive]
RANMA-SLYR 'Old Soul' by Francis Bourque - [Dld Date=12/31/2000] - [Dld Size=202kb] - [Dld File Cnt=6] - [Dld Status=Inactive]
RANMA-SLYR 'Ranko Inverse' by Ghent Johanson - [Dld Date=12/18/2003] - [Dld Size=24kb] - [Dld File Cnt=3] - [Dld Status=Inactive]
RANMA-SLYR 'Ranma Inverse' by Lina Metallium - [Dld Date=05/22/2003] - [Dld Size=135kb] - [Dld File Cnt=10] - [Dld Status=Unknown]
- - [Direct URL] No known valid link
RANMA-SLYR 'Ranma Mazoku Factor' by ContraBardus - [Dld Date=08/25/2008] - [Dld Size=389kb] - [Dld File Cnt=9] - [Dld Status=Inactive]
RANMA-SLYR 'Ranmazoku' by RedMenace - [Dld Date=09/02/2004] - [Dld Size=484kb] - [Dld File Cnt=8] - [Dld Status=Complete]
RANMA-SLYR 'Redheads' by Kenko - [Dld Date=05/14/2010] - [Dld Size=94kb] - [Dld File Cnt=4] - [Dld Status=Inactive]
RANMA-SLYR 'Slayers What!' by Vahn - [Dld Date=09/02/2000] - [Dld Size=21kb] - [Dld Status=Inactive]
- - [MD Source=FF.NET] - [MD Rating=K] - [MD Word Cnt=3863]
- - [MD Summary= A mysterious group of figures are stalking Lina Inverse Gourry Gabriev and the rest of the Slayers. Who are they and what do they want? And what are the Sailor Senshi and the Nerima Wrecking crew doing int he Slayers Universe! Tune in and find out! DISCONTINUED 9-11-13 Ending outline at the end of chapter.]
- - [Direct URL] https://www.fanfiction.net/s/67794/1/
- - [Author1 URL] https://www.fanfiction.net/u/14950/
RANMA-SLYR 'Soul Mates' by Driver Jim Ohki - [Dld Date=10/21/2004] - [Dld Size=27kb] - [Dld Status=Inactive]
RANMA-SLYR 'The Long Road to Redemption' by ss4-link - [Dld Date=07/31/2008] - [Dld Size=28kb] - [Dld File Cnt=3] - [Dld Status=Inactive - Preview]
RANMA-SLYR 'The Slayers Wild' by Troy Thomas - [Dld Date=11/27/2002] - [Dld Size=108kb] - [Dld File Cnt=5] - [Dld Status=Inactive]
RANMA-SLYR 'Totally Inversed' by Michael The-Zorch Haney - [Dld Date=10/23/2007] - [Dld Size=42kb] - [Dld Status=Inactive - Preview]
RANMA-SLYR 'When Past and Presant Collide' by Laina Inverse - [Dld Date=04/20/2005] - [Dld Size=80kb] - [Dld File Cnt=7] - [Dld Status=Inactive]
- - [MD Source=FF.NET] - [MD Category=Anime X-overs] - [MD Rating=T] - [MD Word Cnt=15865]
- - [MD Summary= Three girls must set out to save the world from one evil person. They are joined by their soulmates and friends in the fight but will they come out on top? SlayersRanma crossover Alternate universe DEAD]
- - [Invalid URL] https://www.fanfiction.net/s/1688454/1/
- - [Author1 URL] https://www.fanfiction.net/u/243424/
EDIT:
There was a older version of 'When Past and Presant Collide', which I think was 22+ chapters... but that was abandoned and rewritten into the above, which in turn was also abandoned...
I also have the 4th chapter of Redheads, the same preview chapter others have already mentioned, so did not bother listing it separately,but my copy came from the now abandoned FFML search engine... it is dated 1/7/2000. Thing is, I never overly cared for Redheads, Shrugs, just never appealed.
There is also the below, but the below was part of a mistaken update by the author that he later removed, and to my knowledge was never re-released on the public site he has:
RANMA-MULTI 'Breaking Chains - 5 Unbreakable Chains 2' by TheGrum - [Dld Date=07/11/2007] - [Dld Size=404kb] - [Dld File Cnt=2] - [Dld Status=Inactive]
Also, not Ranma related, but there was at least one good short Love Hina crossover where Keitaro was raised by Xellos... and took place during the burning blades arc.