A quick check of the St. Columba register prepared by DuncanMacDonald shows the following children of John MacMillan and Christy MacLean baptized there
p 107, 1858, Miles MacMillan, Born 20 June, Bapt 29 June, conc 5 Lochiel
p 111, Flora MacMillan, born 29 Nov 1860, bapt 23 March 1861, Conc 5 Lochiel
p 116, Donald McMillan, born 19 April 1863, Bapt ?, Lot 18, Conc 5, Lochiel
p 121 Lilly McMillan born 7 Feb, bapt 11 Mar 1866, Rev Hugh Lamont, Minister, Conc & lot not given
p 128 Harriet McMillan born 13 Sep 1870, Bapt ?, Rev Alexander McKay
for the 1879 Glengarry Atlas. Land records are available from Sir John Johnson House for a small fee firstname.lastname@example.org
. The index is available and the actual document copies after 1867 - before that date you must go to Ontario Archives.
As for McLeods - you will need the book MacLeods of Glengarry (I don't have a copy with me) and a good idea of where they lived in order to connect with the right family. There is also an extensive MacMillan genealogy - best to contact the clan society - although I have seen it at the National Archives of Canada so a good library may have a copy on hand or will get it for you.
If you are not from Glengarry, please be aware there are hundreds of MacLeods and MacMillans to search through, so the more information you have about these families, the more likely you will find the correct one. Fortunately, many have been documented.
All the best