According to Whitaker's "Words"-programme both "nimium" and "nimis" are adverbs. I cannot furnish any sample sentences, but two entries in the dictionary "Vocabulary of High School Latin" by Gonzalez Lodge may be helpful:
1151. nimis. Caes.—(vii. 36, 5). Cic. i. 3; iv. 13 bis; P. T, A. 28. Adv. too, too much. Cf. nimius.
1152. nimius, -a, -um [nimis]. Caes.—(vii. 29, 4). Cic. i. 10; III. 7; 10; 22; P. 23. V. IV. 657 (ties). Adj.too much, excessive. Nimium as adv. = nimis.
According to Lodge, at least, nimium
can be used interchangeably as adverbs while nimius, -a, -um
(including a different type of nimium
) is used as an adjective. I hope that I interpreted this correctly.
I hope this helps,