I' m sorry, but your setup doesn' t seem that exceptional to me. Professionals and/or experienced support specialists shouldn' t have an issue here.
But as Ede already said: you shouldn' t buy 620B' s without configuration if you' re not experienced with Fortinet (and sorry to say so, but if you have to ask for the difference between routed and transparent mode, you' re not).
I would stronly advice to get a decent professional to set this up for you. Like I said before, your configuration shouldn' t be too difficult. I have had a few hundred devices in my hands and out of experience this shouldn' t take more then 2 days to set up (although I should know your environment and all your requirements for a better estimation).
Kind regards,
Willem
Willem
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FCNSP (Fortinet Certified Network Security Professional)