St Juste, 30, excelled during North Ferriby United's run to FA Trophy success in 2015 and played for the Pilgrims` league rivals AFC Fylde and Bradford Park Avenue last season.
He has penned an initial one-year contract at The Jakemans Stadium.
A St Kitts & Nevis international, St Juste played for the likes of Darlington and FC Halifax Town earlier in his career.
Dibble, 22, meanwhile, ended last season with National League South side Chelmsford City after beginning his career at Barnsley.
He also has first-team experience following a loan spell at Nuneaton Town.
The commanding stopper is the son of former Manchester City keeper Andy Dibble.
Both players trained with the Pilgrims for the first time on Tuesday evening.
(Photo shows Christian Dibble welcomed by manager Dennis Greene)