Well, for Toxapex in the story, I'd say a great moveset is Venoshock, in addition to Recover, Toxic Spikes, and Haze. Venoshock deals more damage when the target is poisoned, which pairs well with Toxic Spikes. Recover, of course, is there to keep Toxapex alive. Haze can be used to clear out any stat changes on the field, which can be really handy in certain story battles.
One effective moveset for Cofagrigus could be Shadow Ball, Will-O-Wisp, Protect, and Toxic. These moves give it a mix of offensive and defensive capabilities.
One common theme is adventure. Since Gohan has his Beast form, they might go on adventures together fighting enemies. Another theme could be romance, exploring their relationship deeper.
Well, another place could be some dedicated Dragon Ball fan forums. There are often sections where fans share their own fanfictions or recommend good ones. Some popular Dragon Ball forums might have threads specifically about Videl and Gohan Beast's relationship in fanfiction form. You could also ask around in those forums if you can't find what you're looking for right away.
In this fanfiction, Gohan might learn valuable life lessons from Grandpa Gohan. Maybe Grandpa Gohan would teach Gohan some old - fashioned martial arts techniques that have been forgotten over time.