A hythe or hithe is a small harbour or port. The site of the present Findochty harbour is shown on the first edition of the Ordnance Survey six-inch map
http://maps.nls.uk/geo/explore/#zoom=16&lat=57.69916&lon=-2.90324&layers=B000000000F as a small bay named Broad Haven. Sea walls were built in 1883 to turn it into a harbour. You can use the slider labelled <> at top left to switch from the mid-Victorian map to the present-day satellite photograph.
I speculate that Broad Haven and Broad Hythe are synonymous. You would hardly have both a place named Broad Hythe and a place named Broad Haven in a place as small as Finnichty. If so, then it suggests that your Fletts probably lived in one of the cottages on the shore to the south of what is now the harbour.
You could start with the 1878 Valuation Roll and trace back in the earlier VRs until you find that the Fletts 'Cornal' are described as tenants rather than proprietors. Then go to the registers of sasines and find the record of the sale of the house to a Flett 'Cornal'. Once you have this, you should be able to get an exact description and plan of the property, which will allow you to pinpoint exactly where it is.
The earliest VRs are about 1855-6, but the oldest ones don't list every individual house. If the Fletts 'Cornal' bought the cottage before the earliest VR that does list houses separately, you would have to search the indexes to the Registers of Sasines year by year. I do not know whether the Registers of Sasines would include tee-names.
According to the Statistical Account of Scotland, Findochty belonged to Lord Findlater. He was one of the family of the Earls of Seafield. The Seafield Estate papers are in the National Records of Scotland, catalogue GD248/ (I gather there are two-and-a-half tons of them!). They should contain records of rents and possibly estate plans which might or might not show who rented which cottage or tenement. ('Tenement' here in the sense of a piece of ground rather than in the modern sense of a block of Victorian city flats.)
If you are going to do this, it might be useful first to extract from the various census a list, in order, of who lived in which house in Broad Hythe in which year, and try to work out in which decade a new tenant came in. Then try to match the changes of tenancies with the changes in the VRs and census. I have done this for a similar small fishing village in an effort to work out the succession of occupants for each house - but I only had to work with about a dozen houses, and I know that it was only built in 1852 because there is a coat of arms on the building telling me so.
The 1851 census of Findochty is singularly unhelpful - the addresses are all 'Fronting the Sea' or 'Road leading East', 'Road leading south' or 'Road leading West', 'Leading to the Brae', 'Fronting the North', 'Opposite the Shore' and 'Road leading to Shore'.
If you want to get back still further, you will have to look elsewhere. Findochty was founded in 1716 by fishermen from Fraserburgh, but the Fletts are said to have come there from Shetland. There is an excellent Shetland genealogy site at
http://www.bayanne.co.uk/Genealogy/Gene.html Good luck! I suspect you may need it - and that you will need either lots of patience and determination, or the services of a professional Edinburgh-based searcher.