Goldfish are not livebearers so they are never truly pregnant. If you think your goldfish is pregnant than she is just getting ready to spawn by becoming full of eggs. Unless there is a male goldfish present the eggs will be unfertilized and will not hatch into baby goldfish fry. Your female goldfish will soon lay her eggs, and if no further action is taken, she will then eat them. As for the small tank, I would really recommend moving her into a bigger one.
No, first of all they should not be in the same tank, and secondly, goldfish are egg laying fish so they do not get pregnant
Goldfish do not get pregnant.
Small or Big it should be kept in a huge tank with Filter and Oxygen 10 gallons per Goldfish At least
Yes, goldfish are very messy. Goldfish need 10 gallons per fish, you actually need a 20 gallon tank or about a 70 liter tank for 2 goldfish. But the small tank will do while they are small, you will need to eventually get a larger tank.
For twenty small (1-1.5 inch) Goldfish you should have something like a 30 gallon aquarium.
You should not have goldfish and platies in the same tank ever.
If the goldfish is a fancy variety, it can live alone in a 20 gallon tank. Goldfish do not belong in tanks any smaller than this as they grow very large
It is important to remember to put things in a goldfish tank, even goldfish get bored. Some of my suggestions for tank decor are artificial plants and a "hidey hole" for your goldfish.
Goldfish eggs should be clear with a small dot in the middle if they are fertilized. If the egg is entirely black if may be growing fungus or mold and should be removed from the tank.
Well it'll help if it has water in the tank?
None. A single goldfish should have a minimum of 75L. You would be better off keeping small tropical freshwater fish.
Since an oranda is a type of goldfish, they will be fine together