I am thinking about adding new rifles and replace units in my space map, but I am scared that the effects (explosions, exhaust effects, etc.) will begin to disappear, because of the amount of new effects.
Actually I am wondering, if the stock rifle, which is given to the stock marine takes as much effects as a rifle I would add, with maybe bigger explosions and sparks but not more effects.
I hope I did not explain this in a too confusing way guys.

The exact reason for effects disappearing is hard to pin down. It seems it's a particle count issue, but the particle seem to stack as well meaning the game doesn't dump memory very well if at all.

Bottom line, more units = more effects = more particles = particles disappearing.

So yes. I suggest just focusing on the space aspect of space maps instead of worrying about ground combat.

But what if I replace a small explosion with a huge explosion. Does that matter or change anything?

While the size of the effect does not matter or contribute to the particle count, larger effects do typically tend to have more particles than smaller effects

ok good that answers my question. Is there a bar or table in the particle editor where I can see the particle amount?

Yep: (see around where my mouse cursor is located)