Paul then speaks of then making himself like someone in order to gain them for Christ, however not to be like them in sinful passions. When he speaks of weak he doesn’t mean that way.
Next we have the word “save some”. We will not be able to get all saved, but we need to get some saved. I speak to all Christians if we cannot get some to be saved what good are we. We might have just gone home to be with the Lord.
Paul speaks of keeping his body under control because after preaching to others he didn’t want of himself be disapproved as a servant of Christ. This doesn’t have anything to do with salvation by grace.