Graduate Fellowship: Who to ask recommendations from
Recommendations are only helpful when they come from people that know you well. In particular, your letter writers should be able to give an informed opinion on how you meet whatever selection criteria are relevant for the fellowship, for example they should be able to comment on your academic achievements and potential. Teachers with whom you have not interacted yet, but will only do so in the near future, will not be able to do that, and will likely decline to write a letter of recommendation.

Of course you certainly would not want a bad letter. I would therefore to suggest to specifically ask your letter writers if they will be able to write a strong letter of recommendation. If they think they won't be able to do that, they will hopefully tell you to ask someone else, instead of writing a bad letter.