I think these two signings are exactly the right sort of move for Atlanta. They have no long term risk. They payroll figure in 2017 is mostly irrelevant.
IF they don't work out they can be cut.
If the moves work out and they contend then that's great. If they work out and they don't contend they might be trade assets.
If they work out and young players also force the issue, they can be traded for assets or they could go with a six man rotation.
They keep their draft pick, their prospects and there is a decent chance they upgraded the rotation without committing long term dollars to middling starters who might prevent them from spending money when it matters.
Please note, that I don't think that it really makes them strong contenders for anything, but I also think that Atlanta is not ready for that step. I don't think trying to force it is wise.