Hi and welcome. Sorry your first foray into fish keeping is not going well. Let's look at the problems one at a time...
Prob # 1 - after about 2 weeks of moving them into their new home, they started to go to the top of the tank i though it was because they were hungry then checked at the petshop and they said they might need some air so told me to buy oxygen plant which i did and put it in their tank.
A bowl is totally unsuitable for goldfish (they have been banned in some countries) and they need something much bigger. Personally I like to see all but the fancy long finned varieties in ponds, not tanks or bowls.
If oxygen lack was the problem (and it probably was) an oxygen lpant would have done little to alleviate it. An airstone would have been better.
Prob # 2 - after doing a full water change and putting the new filter and airstone in the water became blacklike and murky so i assumed i did it wrong and after 3 days did another full water change, then again after 3 days noticed the water changed to the same colour and decided to pull the filter out. I believe this to be the problem and decided to just do a 10% change everyday to keep them happy
As suggested, did it have carbon in it somewhere? Sounds like loose carbon dust has fouled the water. What sort of filter is it? If it was one you put under the fravel, did you clean the gravel? If not, you will have stirred up all the muck trapped in the gravel since you had not been cleaning it before this by the sounds of it.
Prob # 3 - before the last water chage i notice my fish were really fat, alot fatter in the tummy than when i got them so jumped on the net and realised i may be over feeding them so have now cut down the amount of food i give them especially cause they live in a bowl!
Some species of goldfish are supposed to be fat. Feeding once a day, a small pinch, is plenty. More important is to make suer there is no leftover uneaten food which will foul the water.
Prob # 4 - after noticing the large bellies - i notice one of my fish has two white spots on both sides of its tummy and am now worried it is sick.
Do you mean it has 2 white spots at the front of its belly, behind the gills? If so, this means you have a male who is ready to spawn if given the chance. If further down the belly it is most likely whitespot, caused by stress.
I have done alot of reading and want to get more fish so am investing in a larger tank - hopefully this will make my current two fish happier. After that i want to add another 7 goldfish to make 9 my total collection.
Please do not put 9 goldfish in a bowl or tank. As said, they shuold be in a opnd. Have you considered a different species of fish to try?