what is the sub floor? cement slab?? then you plan is good after getting the glue up
sub floor wood? then you plan is bad, you would have to lay a sub floor of new plywood as per manfactures instructions on tile your laying down
they probally recomend a 1/4 inch underlayment plywood(thats what you ask for a lumber yard) nail the underlayment down with 2 inch ringed flooring nails at 60 nails per sheet ply, lay your self stick tiles on new plywood for long lasting floor
it wont stick to old sub floor and your haveing to do the job again very soon
read instructions for new tiles in relation to laying over old sub floors