Forbes was charged with the rape of a 15-year-old girl earlier this year along with six other men.
As part of the deal for him to sign for Town, his bail conditions have been changed.
Forbes signed for the Hoops three years ago from West Ham, winning the player of the year award in his first season.
Capable of playing at centreback or rightback, Forbes has found himself struggling for first team football in the last year and with Danny Shittu returning to fitness was released by Rangers boss Ian Holloway.
It is believed that Forbes is the defender Slade was chasing last week.
Forbes is joined at the club today by Austrailian striker Paul Harries.