Ive seen many people use double bumpers. Its quite possible it was factory as well, due to the smaller playfield size. It doesnt hurt to add them at all. Just make sure that the posts are well tightened. (which you should re-tighten just about everything on the machines anyways, as vibrations tend to loosen things quickly... and can lead to damages)
As for pop outs... if its happening a lot, you might try adjusting the metal hood angle. By creating a steeping angle, it can force the ball to go in a more direct downward vector. Some people put a small piece of foam on the top of the hood, to help as well.
Also, before that... replace your flipper assemblies. As these get older, they start to wear down. Parts like the bushing, which holds the flippers shaft in place, wear to the point where theres some play (wobble). Once theres play... it allows the flippers to travel in a path that the game was not designed for... and can shoot the ball in places and speeds that it normally wouldnt do. It also can make the timing all off, and vectors all off... so that certain shots that used to be easy... are nearly impossible.
There are kits which contain all the parts needed.. and it doesnt take that long to install them.
Just make sure to lock the flipper shafts down at the correct angles. Theres usually a small spring pin hole right under them. By placing a small pin or hex wrench in that hole, you can align them to that pin. Thats why those are placed there.
Also, its very possible that someone may put the incorrect coil in a pin, which may be too powerful, or not powerful enough. The kit will take care of that too.
A new ball set is a good idea though. Even limited wear, should be seen as unwanted. Clean every part, rubbers (Id replace them all when you can) and under every part... as well as any subways (in certain games). Trapped dust, dirt, metal slivers.. can get trapped inside the wax, picked up by the ball, and drivin into the surface of the playfield.
I stopped using wax, which happens to make more of a mess than anything... and now use F21 (formula 21). Its urethane formula, is really durable & protective... yet it wont flake off nor trap and spread crud all over the place. It will make the field glossy new looking, make it play faster, reduce frictional ball spin wear to nothing, and keep it cleaner longer.
Tested in the arcade I managed, for over 2yrs, with nothing but excellent results, quicker & less frequent cleanings, and higher sales from better faster like-new play, and shinny clean new-look.
Edit:
One thing you should know about Baby Pacman... is that from what Ive heard... they are really tempermental, and hard to maintain working. Im not sure what the faults are... but I believe there are 'bullet-proofing' proceedures, that if done, help keep it running a lot more reliably.
Edit 2:
Almost forgot... I wouldnt change the intended ball size. The game is specifically designed for that ball size and mass. Doing so, will change the entire vector paths that the ball usually uses... and so the game may not play right anymore. Ive played a game with smaller diameter ball, and the feel and reactions are not right. Just does not feel or play good at all. (A good example, are those toy pool tables. The smaller balls, just do not react the same as real full mass balls)
And finally, use the intended color rubbers. Black rubbers are a LOT little less bouncy than white, and so it also changes the way the game plays. Dont forget, to check the machine for optimal leveling. If the level is too shallow, it may one of the problems. Too steep can also be an issue.