What about Gallade? Gallade is the evolved form of a male form of Kirlia, who is a Gen 3 Pokemon?
If that is the case, I will suggest these Pokemon for each gym leader. The ones in bold are the ones I personally would choose for them, the rest are there in case those suit your fancy.
1. Debbi - Bug
-Spinerak/Ariados
-Scyther
-Weedle, Kakuna/Caterpie, Metapod
-Beedrill/Butterfree
-Paras/Parasect
-Volbeat/Illumise
-Surskit/Masquerain
-Venonat/Venomoth
2. Wyatt - Water
-Gyrados
-Lotad/Lombre/Ludicolo
-Goldeen/Seaking
-Staryu/Starmie
-Tentacruel
3. Gallahad - Psychic
-Gallade *if he can be, considering how it nearly matches his Leader's name, and he is the evolution of Kirlia, which is 3rd gen.*
-Espeon
-Alakazam/Hypno
-Kadabra/Drowzee *the opposite of the one picked above, assuming you want this*
-Ralts/Kirlia
-Gardevoir *assuming Gallade isn't applicable*
(Assuming Gallade is not applicable, I would say Gardevoir and Hypno, along with Espeon.)
4. Kai - Fire
-Arcanine/Houndoom
-Growlithe/Houndour
-Vulpix/Ninetales
-Flareon
-Numel/Camerupt
-Torkoal
-Slugma/Macgargo
5. Faust - Dark
-Umbreon
-Houndour/Houndoom
-Crawdaunt
-Absol
-Sableye
-Carvahnna/Sharpedo
6. Zari - Electric
-Raichu *Cliche I know, but it's really powerful, and trainers are powerful, yes?*
-Voltorb/Electrode
-Electrike/Manectric *opposites again*
-Magnemite/Magneton *Perhaps Magnezone, seeings how he is the evolved form of Magneton?*
-Lanturn
-Plusle/Minun
(Again, assuming Magnezone is not applicable, I would recommend Lanturn.)
7. Glacies - Ice
-Snorunt/Glalie
-Dewgong
-Cloyster
-Delibird *well, it certainly wouldn't be expected*
-Swinub/Piloswine *again, maybe Mamoswine?*
-Sneasel
-Spheal/Sealeo/Walrein
(If Mamoswine is a no-go, I would pick Walrein.)
Again, the choices I picked in Bold are what I recommend, while the list gives your the other options. Not every Pokemon from each generations were put on the list, they are ones that I would suggest, but the bolded Pokemon are the three I think would be more diverse and 'unique'.
Happy choosing, and best of luck with your story! Hope I helped! :D