I generally keep one of everything I don't already have, just in case. Sometimes you hear about random uses, like the green armor knight guy from Thriller Bark being handy for coliseum fights.
That said, I'd sell one of the STR Sanjis for points, rather than waste it on maybe a skill up and maybe a good socket. Other than that, they all have good uses in specific teams.
If you use non-STR guys with Akainu but they get a red orb, they still get a boost, yeah? I doubt anyone is going to get close to a maxed Arlong with a matching orb, but I feel like Alvida and Diamante could both be replaced, maybe Sanji and Garp if you want to stay all STR?
Eh, I don't think I'm gonna replace Diamante. His atk is only 70 less than Arlong, and unlike Arlong, his Special isn't useless. Unless 70 atk makes that big of a difference (it very well might, but I figured my last two hits will be going to Akainu and my helper captain). I'm not sure how useful it is to have multiple delay units, but having the extra 2 turn delay (along with GPU) and decent damage every turn for 5 turns (think it's 70k per turn at max level).
I'm definitely replacing Alvida with TS Sanji, since like I mentioned, he seems better than Alvida in every way, other than 70% dmg reduction instead of 80%, but I think the 1.75x to slot effects way more than makes up for the 10%. Not to mention his +50 to all stats Crew Ability.
I think if I'm gonna replace Arlong with a pure beatstick, it would be TS Chopper, since his attack is a tad higher and his special is better. He also retains his STR slot as long as his attack is Perfect.
But yeah, on Garp, I'm not sure. Obviously he's great for princess turtle runs, but other than that, I'm not sure he would help much.
Thanks for the tip on keeping one of everything (I made sure to check on the trash units, and none of them had any useful niche abilities). Sadly, I had already merged the two Sanji's. Which thinking about it, I guess was stupid (remember, still mostly new
).