Yeah I don't think the level 6 prisoners are much ahead of Burgess/Auger/Lafitte/Doc. I'm sure BB would've loved to have gotten a guy like Croc or Jimbe, but they weren't available, so he instead took guys like Sanjuanwolf. Also, just because BB told Auger and Burgess to stand down against Ace doesn't mean they don't have Haki, it's just means Ace would overpower them, which isn't saying much since pre-timeskip, Marco/Jozu/Ace were without a doubt stronger than those guys. It's basically implied that all Yonko are approximately equal and if BB's crewmates were significantly weaker than WB's commanders, Marco would be a Yonko, not BB. You also have to realize that during the timeskip, BB has been actively conquering the WB pirates' territory. Does anyone honestly think no major battles broke out between them? BB can't take on every WB commander at the same time, so it's safe to assume that guys like Burgess were taking on the others and probably had the upper hand as well. Even if we assume that WB held off both Marco and Jozu, Burgess was still able to take on Curiel level fighters, which means he could legitimately take on Marco/Jozu. And Chou, being able to fight someone on equal terms=/=equal strength, Marco/Jozu>Burgess is a given, but Burgess would almost certainly be able to hold his own, or else the BB pirates would get flattened. Jozu can take on Aokiji, but Jozu would also lose 9/10 fights.
As for growth rate, yes, Burgess absolutely grew as much as Luffy/the rest of the Strawhats during the timeskip if not moreso. Based on what we've seen in the manga so far, training is not the fastest way for someone to get stronger. Real fights are how someone gets stronger. While the SHs were busy training in remote parts of the world, Burgess/Auger/Lafitte/Doc were out there fighting actual high level opponents and getting experience for using their techniques in practical situations. The timeskip was just a plot device for Oda to set up Blackbeard/Akainu/Kidd/Law etc in the new world, give Luffy Haki without him suddenly waking up one day a master of CoA, and giving guys like Usopp/Nami/Chopper new techniques so they don't get fodderized by everyone and everything in the New World. There's no way that a guy who'll likely be a final opponent for a Strawhat would be weaker than Luffy halfway through the story. He's a main fighter on a Yonko ship for heavens sakes. If guys like Pekoms, who is supposedly a "second tier" crewmate of Big Mom is that strong (seriously the guy could legitimately take on any of the Monster Trio just with what he's shown so far), I have no doubt that Burgess would win 1v1 against any strawhat.
As for Don Chinjao/Sai/Boo's numbers, could they perhaps be numbers in Kaidou's zoan army? The first one to get an artificial fruit was "experiment #1", Android 18 (yes I'm calling him that from now on :D) was "experiment #18" etc. Although more likely than not they're power levels or district numbers. Speaking of Android 18, highly doubt he beat Drake. At most that fight ended up being a draw. According people on Shabondy Archipelago post timeskip, Kidd and Drake have been the most successful Supernovas. Getting a beatdown by basically an Impel Down Demon Guard on steroids can't bode well for your reputation, even if Android 18 is a Curiel level fighter.
Just for fun:
Blackbeard vs Luffy (obvious)
Shiryuu vs Zoro (need to cut diamond somehow)
Sanjuanwolf vs Jimbe+Sanji (Wadatsumi was a foreshadow ;))
Pizarro vs Sabo (fap)
Vasco vs new strawhat?
Catarina vs Nami (female vs female)
Burgess vs Franky (brawler vs brawler)
Lafitte vs Brook (they're both skinny, half-dead, and dance around a lot)
Auger vs Usopp (sniper vs sniper)
Doc vs Nami (I honestly have no clue what's going to be done with Nami)
Stronger vs Chopper (jk he'll end up fighting either Pizarro who has an "animal" sense to him, or Doc in a Dr vs Dr fight)