Well, it seems as though you have two separate rooms, one for vegging, and one for flowering. Is that so? If so, and you can't move your vegging plants to the flowering room and you don't have another hps, then I'd say that you have no other option than to use your fluorescent bulbs for the time being.
Fluoros for flowering does work, especially if you have lights with the proper light spectrum for flowering. If it were me in the same situation, I'd tie those plants down to the desired height, and if I still felt it necessary to put them into flowering, then I would put them into flowering using the fluoros until there was room in the flowering room for them. However, one little problem: your plants will get quite a bit taller for the first few weeks of flowering. Yep, I think you might want to tie them down.
I hope you find this helpful,
Alistair
