Sure you can, there is even challenge, including in Contigency Contract, that require you to use only 6 Operators, so you can deffinitely use 1 medic only. (And I managed to pull off some map with 5star and 4stars operators so it’s not “you can but only with busted 6stars op” )
I’m not a pro by any mean so take my oppinion for what it worth, some people might have far better idea but,
I would recomend of course Silence as a healer, since she has a Drone so you kinda have 2 healer in 1, but of course as a 5★ it’s hard to get her.
You already have Gummy and Matoimaru so it’s nice !
I suggest you to upgrade Matoimaru, because her ability self heal her up to 40% when upgraded to 7 (and even 50% on Elite 2).
Gummy is really good too, I’m actually using her in CC so I think it’s a good operator (even tho’ there is the same version in 5★ like Hung , but of course that require luck to pull it lol)
As a Deffender in the 4★ bracket, you have Cuora that has a self heal (while still blocking but not attacking) as a 2nd skill.
The rest is up to your “pull luck” I guess.
You can visit " aceship(dot)github(dot)io " (i can’t post link sorry) it’s a very usefull tool for Arknight that provide information regarding Material, Possible drop in Recruitment, and a list of all the Operator by bracket, which would let you seek operator that can self heal or heal other while attacking !
Hope you will have fun.