Here's a discussion going on entrance reducers right now. Like everything bee related, lots of opinions.
https://www.beesource.com/forums/showthread.php?354165-Entrance-Reducers&p=1721975#post1721975
"Installed my nuc...." Does that mean you brought it home and set in it's intended location or you transferred the frames and bees from the nuc to a Deep? The bees you see "trying to enter the hive" if they're having difficulty, guard bees may be repelling robbers. Lots of nucs out there, if the "smallest opening" is an inch or two, wouldn't worry about it for awhile. Doubt you have a major flow going and a new nuc hasn't had time to build up a big population. One of the big concerns with a new nuc or package is robbing, smaller entrance is better IMHO.
Lots of very experienced folks only have small openings all summer compared to other very experienced folks.

Personally I ran my hives with only a 3" opening at the bottom entrance year round, only change was adding mouse guards for the fall and winter.
To perhaps misquote Michael Bush, "If you don't know what to do, don't do anything." I'd reflect on that.
Good luck,