Striker Hinds arrives from Luton Town, while giant defender or midfielder Plavotic – who has spent a lengthy time on trial at the club – arrives from Austrian club FC Pasching.
Head coach Lee Johnson said: “Freddie is someone we were close to signing in August and we’re delighted to get the deal over the line now.
“He’s almost in the Nicky Maynard mould with his attributes. He’s got a good centre of gravity and certainly knows where the net is.
“Tin is a big lad and is very good on the ball. We see him as a similar player to Aden Flint and although he’s younger, in terms of his football development he’s probably ahead of where Aden would have been at the age of 19.
“Hopefully he can show the same professionalism and mindset as Aden over the next six or seven years then he could follow in his shoes.”
Hinds recently turned 18 but has been featuring for Luton’s Under-23 squad this season.
Last year he scored a hat-trick in a 10-2 demolition of Dagenham & Redbridge Under-18s and then hit another treble against AFC Bournemouth en route to Luton winning the South East Alliance Under-18 title.
In the 2015/16 FA Youth Cup competition, Hines scored goals in the first three rounds of the tournament as Luton overcame Colchester United, Leyton Orient and Notts County, before losing to Blackburn Rovers in the quarter-finals.
As for Plavotic, 19, he was born in Austria but has Croatian parents and has featured twice for the country’s Under-19 side.
Although on the books of Pasching, Plavotic most recently spent time on loan at German club Schalke 04. He can operate in central defence or as a holding midfielder.
Both have signed two-and-a-half-year deals, with a year option, and will link up with Jamie McAllister’s in-form Under-23 group for the remainder of this season.