Customizing fonts?

I don't understand what you mean by customizing fonts. Are you talking about modifying letter shapes in a vector graphics application like Illustrator? If so, you can do that with any fonts, but it usually involves converting the type object to raw vector outlines in order for the letter shapes to be modified. That kind of work is legal to do since the letter glyphs would just be raw shapes. The vector shapes would no longer be part of a digital font file.

If you're suggesting modifying existing fonts and then trying to redistribute altered font files I don't think that would work with any fonts hosted by Adobe Fonts. Even the fonts that can be downloaded freely from Google Fonts have certain legal protections.