So... Basically if you want to get the ambrosia you are going to have to have at least 2 different Sims: One, the Founder who can only max out one skill, be it cooking or gardening, and you need another to max out the other two skills.
I guess that this would be a question relating to the first: Your immortal Sim cannot max out a skill so long as he has not supermaxed it?\
Welcome to the forum n8thegr8!
An Immortal Sim can max any skill(s) they want, the only catch is they cannot max a skill that has been or is to become the super-max skill of another Immortal. So yes, your founder can max cooking, fishing, and gardening if you want, but then no other Immortal can pick any if those three as their super-max skill because it has already been maxed by another Immortal. All 8 Immortals have to have a unique super-max skill that no other Immortal has maxed.
As for needing another Sim - mostly just because of the amount of work to be completed before your founder dies (remembering that he/she needs a portrait before aging into an Adult) and to provide an heir.