To your first question, most duplicate Rare Recruit units including Legends, unless they have a limit break which is a mechanic that's only just been introduced in Japan and that we still don't have. This mechanic works by feeding dupes(of the same evolution chain) or a specific item that can be farmed(but is a rare drop). While Akainu is limit breakable, you wouldn't feed him dupes once this mechanic is out unless you're swimming in Rayleigh Points and would instead farm the item. Notable limit breakable units are Raid Mihawk, Raid Zephyr, 5+ Luffy, and Raid Kizaru. There are other RR units that can be limit broken, but those are all of the ones you'll likely find relevant. That said, you should still go ahead and farm up your raid units even without limit breaks, or at least get one copy of each of these to act as class-based captains for when content is restricted to only one class.
As for the second, sadly nope. You'll eventually get a good grasp on guesstimating how much exp you need, but there's no indicator for that.
Since you have TS Sanji, you've got orb boosting covered, and will likely need a class/type booster to get to your full burst potential. The good news is that you'll have plenty of F2P options, but the bad news is that they're at the end of a very long tunnel. QCK Story Hody(he has a branching evolution) is a 1.75x Powerhouse booster on a really low cooldown but to get him, you have to reach Fishman Island in the story mode, Coliseum Zephyr is a 1.75x STR type booster who's next return to the coliseum is up in the air, Coliseum Nekomamushi is an amazing 1.75x Powerhouse booster that also gives full matching orbs against an enemy with delay immunity(this is very common), but is really far away from dropping on Global, and Raid Sanji is a 2x Powerhouse booster but is far away from dropping on Global too.
But tbf, you probably won't need a full team class booster for a while anyways and if you do, Akainu's 50+ cost boost should work out nicely for a pretty damn long time anyways. Pretty sure you can beat Doflamingo with double Akainu, Alvida, GPU, and TS Sanji for an orb boost, no sweat at all and from there, even if Akainu (somehow) can't manage something like Raid Enel, Mihawk + Doflamingo will sweep everything up until the early levels of the Coliseum. From there a 5+ Luffy Free Spirit team takes over for everything else once you have Raid Boa and Sabo. Assuming in some bizzaro world, Akainu can't manage.
EDIT: Alternatively, hoard gems for the Amazon Lily batch, and pull Sandersonia + Marigold. And then, you can clear everything.