Myth’s don’t die fast ... Otherwise there would have been a ban on “mythology” already

....
Yeah it is not exactly stated anywhere as the criteria..my mistake... Also govt favours and see that all shichibukai’s feel comfortable and govt extends their help and support for that... For e.g. Dofla’s businesses ... hancock’s treaty ... Law’s independence to move through govt old facilities... BB’s moving around govt facilities.. etc... And Sengoku indicated once that the pirate should be well known and a threat enough to scare away other pirates (here-
http://www.mangareader.net/103-2341-15/one-piece/chapter-234.html) ...
For the crew thing.. it can easily be misinterpreted given the reason that no shichibukai has been seen with his crew all the time like other pirate captains.... Boa hancock is also referred as pirate empress rather than a captain... We don’t know anything about sun pirates till date.. is it serving under Jinbei or not.. All we know is that Jinbei took the shichi position and got Arlong and crew out of the jail in return and part of sun pirates became arlong pirates , part of it became macro group.. and rest remaining are under Big Mom’s wings... Croc was having a secret organisation under him which was not known to the government.. And as for Moria.. Zombies are to be considered as part of his DF power rather than actual crewmates.. He did have underlings I must say.. And doflamingo is more of like ..different organisations/businesses under him and he has appointed leaders to take care of it and report to him...
During the Marineford war when the shichibukais were summoned to lend their power.. clearly I don’t see any shichibukai was there with his crew or underlings... not even doflamingo for that matters... so clearly govt does not see their crew as part of their powers...
As far as buggy is concerned... My memory is failing me.. but what I can remember is that Buggy knowing Whitebeard .. He being part of Roger’s crew (or was it stated that he was on Roger’s boat and didn’t mention crew) .. He knowing Shanks and referring to him as rival was all seen by the ID escapes only and not the marines.. because whenever all this happened he was standing at the side of pirates and not between marines... But yeah he did broadcast some of the things ... I think I just need to go through war arc again..
Also I would say the marines must know about the buggy’s feat (funny) in East blue and how much of a threat he actually is

.. Like how Tsuru was knowing about Lafetia when no one else knew about him.. Just an infamous policeman ..
And if buggy has been identified as roger's crew mate, wouldn't govt will try to annihilate him rather than offering him a shichi position.. They didn't try to offer that to rayleigh for sure... And even Tom was imprisoned for helping out Roger.. Any person related to roger is just seen as a threat rather than a helping hand by the government...
At the end of day it might happen that before leaving marines.. Sengoku knowingly trolled the govt by suggesting Buggy for shichibukai's seat

... Big troll