There is also the following info in the 1851 census. This is interesting as you will notice the surname Johnstone mentioned again. In the 1871 census 11 year old Charles Small was living with Ann Small aged 41 and a Janet Johnstone (I think it's Janet) aged 49. With the information that Leonard and Ann married in 1853, the reason why she is named Small in 1851 is a bit confusing. Not sure where to go from here. This family seems so messy!
I think this family is a red herring, at least it is at present. There could, I suppose, be a link further back on the Small side.
It's quite normal for a married woman to be listed in the earlier census under her maiden surname. This is the family of George Small and Isabella Johnston.
I also happen to know that their daughter Ann, aged 23 in the 1851 census, could not be the Ann Mackay who married Leonard Small, because Ann Small, daughter of George Small and Isabella Johnston, married my 3rd cousin 4 times removed, Joseph Moug, a few weeks before Leonard Small married Ann Mackay. Joseph and Ann had 10 children, and died in 1882 and 1900 respectively, all in Glamis.
I suggest that you dismiss the idea that there is any close relationship between your Smalls and either of these Johnston(e) ladies, as I think that pursuing that line of enquiry is only likely to muddy the waters of your research.