I'm not the guy to give advice on tropicals. Perhaps it varies with variety, but F. retusa is routinely defoliated in order to produce ramification (and for show aesthetics). One of the better places to get advice on ficus is from Jerry Meislik's webiste
http://www.bonsaihunk.8m.com/ ;-) He knows his stuff.
Here is a picture of a ficus that Kenji defoliated and styled for N.E. Bonsai. There are a few little leaves left, but not for the sake of health.
Kind regards,
Andy Rultedge
www.bonsai365.com/
zone 8, Texas