I'm curious about what the general strategy people take for this game regarding size of girls you actually level up, when you start worrying about leveling or making money, and maybe if there's a route I want to avoid the first time around. So basically, not tips as in a list of stats and items, but general advice.

Give Yuela Prevail III, and you won't need any strategy. Also Critical Snipe for Sewari.

I've been max level for too long to remember when you get them, but Serawi's Enhanced Battery is OP (range 9)
Yuela with Preemptive 3, Vorpal Counter 3 and evade 3 is a BEAST
Wil with any Omni weapon can kick all sorts of ass.

Everyone's unique Accessory gives them a bunch of abilities, including Double Action, so it's definitely worth trying to get them.

Hey guys thanks for the responses. My last question would be after you pick your route, how many girls do you keep leveling up to get through maps efficiently?

One more tip...
You recover HP/SP/FS while not deployed, so you can rest troops that way if you need to. Unless you are actively trying to guard a deployment point, there's no reason NOT to withdraw when you take one over, since you can redeploy one square away from the point. Hey, free movement point!
Obviously you don't withdraw if the enemy will take over the room.

Second promotion.

Making some teleport stones may help for positioning, it allows you to move your team around to safer spots after they attack, reposition for spells without needing to move(lol aoe spells? =D) and perhaps most importantly, toss Aht allllllll the way in front when she has terribad movement.