Glasgow flyer Hughes has two Scotland caps, while London Irish full-back Tonks made the last of his six Test appearances in 2015.
Maitland damaged ankle ligaments in an English Premiership defeat by Wasps two weeks ago and missed the London club's European Champions Cup final win over Clermont at Murrayfield last Saturday.
He will also miss Scotland's upcoming tour of Singapore, Australia and Fiji.
With Stuart Hogg selected for the British and Irish Lions series with world champions New Zealand, Maitland was in line to fill in at fullback for Gregor Townsend's first match as Scotland coach against Italy at the Singapore National Stadium on June 10.
But following Maitland's injury, former Scotland playmaker Townsend has decided to bring in two players to provide cover across the back three.
Glasgow flyer Hughes has two Scotland caps, while London Irish full-back Tonks made the last of his six Test appearances in a 2015 World Cup warm-up match against Italy in Turin.
"We've watched Greig closely this season. He's done really well in a good London Irish side and played a lot of full-back but also covered at centre and stand-off," said Townsend.
"He's a player that brings aggression and a really good kicking game but also has the ability to step up at first and second receiver. Primarily we're looking at him as a 15 but it's positive he can cover other positions as well.
"Rory has had a really good season. He was on excellent form before the injury he suffered just before Christmas and got back to that form towards the end of the season."