Hmmm. Lots of interesting choices here.
First off, to answer your one question: if you put them 10 wide, then Seaguard can fight in five ranks (2 ranks normally, +1 for spears, +1 for martial prowess, +1 for horde). That being said, I ALWAYS find that spearmen coupled with archer units perform better than Seaguard. More units=more mobility. Plus, you are never "confused" with what to do with your units; spearmen move out and protect your archers and act as the "anvil," while archers sit back and whittle down enemy ranks. I've never figured out how to effectively use Seaguard, and they're too expensive to waste.
As for your other choices, I like the Archmage/Mage combo, although I don't see the point of either the Silver Wand on the Archmage (do you really need 5 of the Life Spells?), or the steed on the Level 2. I assume that the Lvl 2 is there for Dispel Magic and using the scroll; why not just drop him in with the archers or in a bolt thrower crew to keep him hidden?
I never take Princes, especially not in a battle under 3000, so I will refrain from commenting on him.
Finally, Putch is right about your Swordmasters getting shot to pieces. You will definitely have to prioritize getting Shield of Saphery off each turn to protect them. You might want to consider either the Banner of Arcane Protection for them, or even dropping your chariot and plumping for the Banner of the World Dragon on your BSB and putting him in there. I know it's a lot of points to trade and it leaves your BSB a sitting duck, but at least your Swordmasters will be guaranteed to get into combat, where they'll kill a lot when used as the hammer with your Seaguard.
You're going to have a doozy of a time against anyone with cannons. Good luck!