is it bad to tighten the screws on the pegs really tight?

Yeah, you won't be able to turn them....only kidding...Just tighten them as much as needed....
 
Don't over do it though
 
Yes, you CAN overtighten some tuner screws. Most friction tuners have plastic heads and you can crack them if you tighten them too much. Depends a bit on the head design, but the cheaper the uke, the more likely they will crack. Geared tuners seldom have this problem, however.
 
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