You can second seal without any penalties at any level. It's going to calculate it the same way no matter what (adds [level - 1]/2 to your internal level). And if you're going magic heavy Dark Mage is a decent choice so you can have another dark magic user early, but you will want to hit Sage for sure for Tomefaire. Dark Mage skills are kind of lame, so you might consider going Tactician -> Mage -> Sage -> Sorcerer. Maybe others too depending on how much skill grinding you intend to do.
Cavalier is good for Chrom since he can use Discipline to build his weapon levels, and promote to Paladin for Aegis which is probably his best non-Lord skill. Archer class tree is useless for him, and unless you want Dual Guard+ there's not much point in going to Great Knight either.
You can only promote to specific classes from each base class. Grandmaster is exclusive to Tactician. If you're level 10 in a promoted class you can reclass to promoted class too, though.
For the classes I posted above, I'd go something like Tactician -> reclass to Mercenary -> promote to Hero -> reclass to Grandmaster.