You should be fine taking just fire and ice on the force tree, and if you didn't mind losing a little bit of damage overall, even grab all three elements, but many are still running fire and ice together. No reason to not have all elements on techer tree, there is more than enough points. Ideally, yeah, you would want a rainbow palette due to element conversion being so strong. If you had the money and resources though, I would just get an Atra 14* rod asap, and until you grab that maybe get a Fire/Light rod, or both Fire and Light. I don't think Fornis was ever really good for combat as force, but it's good to grab the talis to swap back to if there's nothing you can hit and need pp back fast. There's a spell list not that far back actually... but I'll copy paste it here. You should use both rod and talis, but, that's mostly if you're going to bother with Talis Tech Bonus for the extra 20% damage. Some spells suck with talis, like Ilfoie, and compound techs are always better on a rod.
"As for crafts, always just go conc>power=efficient in that order except the following:"