It'll be fine with the hps. Not the correct spectrum to veg but at least it's some light. If you had some fluorescents you could throw those in there too. But if the new lights going to be there in a few days the hps will work. Plant won't start flowering just by using the hps. That's going to take a reduction in hours of light per day to do that.

But you mentioned using some string to tie up the branches. All this will do is reduce their size and strength. If they don't have to hold themselves up they won't get stronger. I've seen people tie them up when flowering, but the stems are as strong as they'll get by that phase so it's needed to help them carry the additional weight of the buds. Take the strings off.