I have 2 farmhouses and 1 Toadstool Farmhouse. I haven't had a single Sphinx spawn from my Toadstool farmhouse. The spawn rate usually balances so I get a Sphinx on my 4th or 5th collection from a regular farmhouse (regular, regular, toadstool; regular, Regular- Sphinx!)
I've tried the sequence backwards too and it still just happens to fall on regular- maybe if I had many more of each i'd notice a different pattern!
For my queendom size, The spawn rate seems more important than type of farmhouse. 😊