Typical bearing retainers don't take up any space, and possibly the worst thing you can do is to add one more ball than there's room for. An overloaded bearing will be very difficult to adjust (often impossible) and will sometimes cause the axle to go off center.

Likewise with changing the ball size. The curvature of the races is such that the balls make contact in the center of the ground and polished area. If you increase the ball size the contact area will move inward on the spindle and outward in the cups possibly going beyond the working area, causing problems.

Leave bearing design to those who do it for a living, and use the number and size of balls your BB was designed for. It's fine to remove the retainer, since it's only an assembly convenience. It might also be OK to add an extra ball, but don't unless you're sure there's room.

You can check the ball count by assembling the spindle into a cup out of the frame and noting whether there's still a gap between the balls. If you use a spoke to move the balls around you can usually get a gap of about 1/2 a ball in one spot. If you can get a gap wider than a ball add one and check again.

If the parts have smooth undamaged races (polished silvery ball track in the middle of the race area) and you use clean, decent quality grease, you should be able to adjust for zero play, plus a bit of preload and have a smooth running lasting bottom bracket.