Im not sure "stainless steel" and "lightweight" should be used in the same sentence.
So powerade can sit in the bottle for up to a year before being sold, and you have no problem drinking it.
But you are not willing to put your water into the same bottle for maybe a few hours to a day at a time and drink out of it?
Yes very minute trace compounds leach out of plastics into our foods. They also leach out of everything we have in our homes, and that we touch. That is simply that hazard of man-made items.
Many studies have been done on blood levels of chemicals that are found in foam cushions, carpets, electronics, etc. Your body is literally already carrying dozens of chemicals from your household environment and industrial environment.
At the low levels most exist at, they dont seem to do us much harm. Every now and then one is made a "poster child" because it turns up in high conc. , esp in breast milk. Whether it was actually harmful, is moot. It gets legislated out, and another new, supposedly better chemical takes it place. Some yrs down the road, the cycle repeats.
My point is, I wouldnt worry about drinking out of a powerade bottle for a few days. Its small pickings.