I wrote the wikipedia bio. and yes Daniela , I think you are right Grand Duke Nicholas the elder was an unpleasant man. He was not reallyt like even by his family.
I could not find information about his illegitmate children with Catherine Choslova. Can anybody add somthing about them ?
I liked your story that's way I've posted it. I didn't know almost nothing about him, so thanks for his story!

On the web I've found this link with geneaology about his children with Catherine Choslova. According to this link they had five children together and first four of them had their own families or at least they were married:
1. Olga Nicolaievna Nicolaiev, b. 10 Jun 1868 - d. 31 Aug 1950
2. Wladimir Nicolaievitch Nicolaiev, b. 4 Jun 1873 - d. 28 Jan 1942
3. Catherine Nicolaievna Nicolaiev, b. 1874 - d. 26 Jan 1940
4. Nicolas Nicholaievitch Nicolaiev, b. 16 Apr 1875 - d. 9 Jan 1902
5. Galina Nicolaievna Nicolaiev, b. 28 Jan 1877 - d. 3 Aug 1878
And the link is
http://www.genealogics.org/getperson.php?personID=I00006699&tree=LEODaniela