If you apply for a tourist visa you must be absolutely truthful about everything. This is the one thing with regard to applying firstly for a tourist visa and later for a spouse/fiancee visa that could seriously jeopardize the outcome of the spouse or fiancee visa.
I read somewhere (probably on this forum) about girl who had a British boyfriend and who applied for a tourist visa to come to the UK to visit her aunt and if I remember correctly the application for a tourist visa was refused However when it came to her applying for a spouse or fiancee visa it was refused because when she applied for the tourist visa she failed to mention that she already had a British boyfriend when she applied for the tourist visa.
Iain.