Whenever new concrete is poured next to old, there should always be an expansion joint between them. There will surely be a crack otherwise because the new material will do some shrinking as it matures.
Can't imagine having a foundation for any wall, especially an exterior load-bearing wall, that doesn't have a proper foundation. Don't know where you live, but if the footings are not below the frost line and not wide enough (check local codes) to support the wall, there will always be movement of some kind, even if you epoxy in rebar! That's also an invitation for critters to make themselves at home.