You can pretty much do whatever you want on "normal" difficulty. It's a good difficulty level for experimenting to find the builds and party compositions that works for you.
1) You've got the right idea with two mages filling each other's gap. Personally, I tend towards the elemental schools, so I have one mage with higher initiative (using gears) specializing in water and earth, casting CC and "set up" spells (like midnight oil, rain, freeze, etc), and the other mage acting as the "cleaner" who cast damage spells that synergize with those elements. On normal, I often finish a battle before slow moving Madora has a chance to smack a baddie.
2) I usually make one of my mage a hybrid rogue, although you can get away with just putting one rank in scoundrel and picking up "Walk in Shadows" (think someone already suggested this). This will allow that character to steal lots of stuff which will help the money issue in the beginning of the game. Add a rank of telekinesis if you want to make it go even faster (or just use telekinesis buff gears). If you don't want to steal for RP reasons, that's fine too. Money will eventually roll in on any difficulty level without stealing anything (I do it more as a scavenger habit of picking up anything that isn't nailed down--traditional adventure games have corrupted me...). I wouldn't bother with pickpocket though. There are a few quest related shortcuts that are available if you have pickpocket, but otherwise it doesn't seem worth it unless you heavily invest in the skill.
3) Since you are not going to have Johan as a regular member of your party, you can just turn him into a crafting mule and have him pick up crafting/blacksmithing instead of any of your characters. It's too bad though, 3 mages are OP (and I quite like Johan's story).
4) My mages use bows/crossbows early game, then once they have a sizable repertoire of combat spells (usually by level 5 or 6), I switch them to using wands. By then, they rarely run out of useful spells to cast on any given turn, and I usually only have wands on them to make use of the wand abilities.
5) You won't need to put more than 2 ranks in lore (if even).
6) CON benefits are retroactive, so you can add CON whenever you want. I personally don't bother with it for my mages, because mages can never have enough INT.
(different story on tactician difficulty though, when so many AI archers have "alpha strike" one shot abilities...)
