Is it okay to reheat something like pizza or lasagna on a hot plate on Shabbos?
Only if it is less than 125 degrees F
Most shabbat hot plates go above 125 degrees. Are those all an issue?
The issue is really the temperature that the food gets to, which is always less than the heating element itself.
To test it, put food on it during the week for fifteen minutes and then stick a meat thermometer into the food.
Does this apply to all other foods as well?
Yes
can you use a hot plate that gets the food above 125 if you put it on a timer and put the food on the hot plate before the timer turns it on?
Yes