White dot, gap, or nub mark? I'm confused.

You can use any oil base marker to cover the nub mark or white dot as long as the colour close to the plastic.
Still can't imagine what gap you mean, a picture might help.

I think I get what you mean now... You use drill to make the female bigger so male peg can fit in nicely...

Don't be surprise, I'm doing aircraft engineering and we always use these terms...
Yes, you can use Anchor paint or other industrial spray cans for plastic although not really recommended because it might make your plastic brittle. But it would be a good alternative for Tamiya spray cans because Tamiya was damn expensive, buy a beginner airbrush set obviously cheaper. My advice is stay away from Tamiya spray cans if you want to save cost. If you don't know how to utilize your money then you can buy it, because with that money I will buy airbrush set.

But then, spray cans and airbrush were just tools, it depends on your need. If you just started painting, you may use spray cans to see whether you like to custom colour or not, if you really like and will do it frequently or you will get into this hobby seriously then I would recommend you to buy an Airbrush set because you can do various type of painting effect with it which spray can can't do or can't achieve same level result.

There are so many hobby paint beside Tamiya, and a lot of modelers using them as well, do it seem weird? Obviously no...  If someone laugh at you if you use spray cans, then you may ask them to show you better paint job that they have did or trying to use spray cans to achieve same effect as yours. But ignore them would be better although this is not my style. If they laugh at your brand, you may ask them to sponsor you Hobby brand spray cans or maybe a high end airbrush set with spare tank. Those modeler who like laugh at other people usually don't have a very very nice skill, they feel proud with their intermediate level in this hobby, there are more skillful modelers that would like to share the proper information in this hobby and you should learn from them.
Cheers...

I always have the same problem with you. I try my best to eliminate the nub marks. After sanding I even rub with Tamiya Compound (Coarse and Fine). It will "somehow" reduce the nub marks but not 100%.