Following the Bristol positional switch, it remains to be seen whether Tandy will continue asking Rees-Zammit to play full-back this summer.
Wales are preparing to face Barbarians on 27 June before Nations Championship matches against Fiji, Argentina and South Africa in July.
The other number 15 options Tandy has named in his 48-man squad for the four matches include Blair Murray, Tom Rogers, Josh Adams and Ellis Mee.
There was no room for more specialist number 15s such as Cameron Winnett, Jacob Beetham or the uncapped Angus O’Brien.
For more than a decade, Wales have been blessed with two world class full-backs in British and Irish Lions pair Williams and Leigh Halfpenny, but the pair have both announced their retirements in the past few months.
A natural long-term successor has yet to emerge, with Williams saying Scarlets back Murray, who has been used on the wing and full-back by club and country, is the best current option for Tandy.
“With the ball in hand he [Murray] can make something happen,” said Williams, who has been working with the Scarlets backs in the past few weeks.
“He has also got a decent [kicking] left foot which is a nice option to have.”
Former Scarlets and Wales centre Jonathan Davies agreed that Murray is the leading current option.
“He [Murray] is working on his aerial game,” said Davies.
“When you look at the options, you have to look at where you are willing to concede in some areas to gain in others.
“Blair gives you the biggest positives in his all-round game for the number 15 jersey.”