Some apps/widgets have options to allow users to define any true type font they please. For Min Text: create a folder on your sc card called "Fonts'. Throw the fonts in that folder, no subfolders. Go into global preferences of Min Text and identify the folder you have all your fonts in. For each widget, define the normal/accented/unaccented fonts as you like.
For Simple Text, installing the app will create a group of folders on your sd card. There will be one called fonts, throw them in there, again no subfolders.
For other apps/widgets (Fancy Widgets, Beautiful widgets, lots others), the design will always show what your system font is. Depending on your phone or ROM, you might have a handful of choices, or not, to set this for the entire device. Rooting your device can give you access to changing the system font; the method for defining this depends on the ROM you pick. I can tell you how to do so for CM7 or MIUI if those are the ROMs you elect. Without root, changing the system font is not possible in most instances, though I have no idea about anything beyond gingerbread. Good luck