The second SSD has nothing directly to do with the first. They do not have to fit together for any particular reason. What you have to make sure is that the SSD fits together with your mainboard.
If Windows and all application programs can be stored on it and there is still a larger amount of free space left, it is ok.
But I would prefer a 512gb version because programmes generally tend to increase the amount of storage space they need. That would be more future-proof.
Basically, the M2 does not need to be specially cooled. With your mainboard, however, you already have a cooling fin including space for the M2 and thus the heat is even dissipated a little better. Perhaps the cooling fin even has thermal paste, in which case it would be even better.
According to the description of the NUC NUC7i5BNH, it has enough space for these SSDs: M.2 2280 or M.2 2242, so you can install this Samsung 970 EVO Plus SSD.