America Story Banner Gacha

They are both story locked so you will have to complete America to unlock them

From my understanding, according to the list of events on the Wiki, there will be three rate separate rate up banners for the E Puribus Unum chapter. The first one at the release of the chapter won't have Cu Alter or Medb on the rate up.
They will both be added to the story summon only AFTER completing the campaign; however, they still wont have a rate up, and I don't think they're available from the initial release celebration banner.
Your best option for Cu and Medb is to wait for their individual banners because they will have rate ups on those banners. That may be a few weeks after the chapter release or more based off of the timing the Japanese version of the game had, but it will be your best chance at them.

If you wait for their rate ups (which you should) then it won't matter if you finish the story first. If you want to roll the story gacha you need to clear the story to get the story locked servants. Though clearing story nodes (and the free nodes after) you get quartz which you can use for more rolls.

This depends on which Banner you are referring to in your question.

IF you completed the story first,
Cu-Alter and Medb WILL be in the STORY BANNER. (But i strongly advice against rolling this banner, as the rates for gettting specifically Cu-Alter or Medb is very very very slim)

Next, both of them will NOT be in the Nightingale Banner that is gonna be on rate up.

However, you can wait a few weeks or so, for the next America SSR Rate up Banners for Medb and Cu-Alter