I honestly havent played Sona that much (only done her once) so can't really answer for her, but she'd prefer to be further in the backline buffing and healing her alies and can be good as the initiator of a fight (her ult). Rakan on the otherhand i can give some infor on (love him xD). He is more of a disruptor/the distraction. He can be the initiator of a fight but it's honestly better for him to be the followup due to the nature of how squishy he is but still needing to get close. My build on him (taking the recent support changes into acct) is windspeakers, Eye of the watchers, Ardent, Redemption, boots (usually mercs) Used to build Locket only do it now if an extra shield is really necessary, Zekes (still a thought not sure if it's really worth it on him for sure but the passive seems to be good sofar :/, used to build it cuz of it's old passive and AP). Deadman's is goodish if you're going against alot of AD and dont wanna overcap toomuch on CDR (usually last item if so). Mikael's is good into strong cc (rarely build it tho tbh.
Rakan can fit into most teamcomps imo (as he can act as the initiator if need be and his kit can be flexible (building more tanky with courage), his laning phase can be abit hard-avg if there isnt much communication/if ur adc is on a different wavelength lol), teamfights and skirmishes are where he mainly excels. He's best if you are aggressive (is a playmaker)