Source being in either a A- or T- pose no longer matters depending on what app your actually using to author animations and the Power tool for doing video games animations would be Motion Builder.
For that matter if you look hard enough you can spot what animation packages has been processed by MotionBuilder and if you look at Mixamo characters and animations takes the naming convention is 100% compatible with MotionBuilders naming convention.
With MB whats most important is not the base pose but matched sets of animations using the same naming convention so that a couple of hundred animation clips can be bulked process.
As a tip real old mo-cap (BVH) the base pose the rotation transforms are normally zeroed out so if you make all the joints 0 0 0 it would would all snap back to the default pose.