Physical DPS seems to be better than black magic early on, unless the mob is vulnerable to a particular type of magic -- which is frequent for bosses. Otherwise they're all pretty even in terms of strength as far as I can tell (~lv28). There's an "official" guide which shows the base stats for each character at level 50:
https://ffrkstrategy.gamematome.jp/game/951/wiki/Home
My go-to party early on was Cloud, Kain, Tidus, Wakka, and Rydia. Nobody had any abilities except Rydia who used Cure and either Thunder/Fire/Blizzard.
Your gear makes the biggest difference in terms of character strength. Each rank (1-star vs 2-star, etc) of weapon is drastically better than the one before, especially when you get to the 3-, 4-, and 5-star weapons. And perhaps most importantly is matching up your gear and characters with the realm you're fighting. For example, Cloud (FFVII) with a 2-star weapon will do more damage than Tidus (FFX) with a 3-star weapon in the FFVII realm.
Getting all the characters (clear normal mode of all realms) is relatively quick and easy. The expert realms are where you'll start getting the better gear/orbs.
You just missed the special event for Tifa and Sephiroth, but you can still get DK Cecil and Paladin Cecil until Apr 16. DK Cecil is pretty easy to get (it's like the 6th level in) -- Paladin Cecil was tough, but doable around level 25.
The game just recently came out, so content is sorta limited right now. Apparently they've been releasing special events each week though. Not sure how often they plan on adding realms/missions. I've only been playing a few days myself. Just starting the expert realms and leveling up my second batch of characters.
Staying free shouldn't be too hard. Just like PAD, everything is accessible as free, it's just slower. Although we all know how disciplined we were with PAD. =P