Do I have to process each font separately?
Nope, x-Tools will check each single font you used in the selected artwork and it will automatically detect the font with the smallest character height. Just use the option "Keep proportions" to keep the ratio between the different fonts and font sizes.
Can I also use x-Tools to check existing artworks?
Sure, just select the textframes or objects you would like to check. Activate the option "Avoid text downsizing" and hit OK. You will get a message in case all characters are larger or equal to the size you defined in the dialog box.
Is it possible to round up the font size?
The calculation of the height is very accurate and you will get font sizes like 6.485 point. Just use x-Tools again and define the target size to 6.5 point. Don't forget to add "pt" in the field in this case.