Two. They need a min. of 20 gallons each.
People most commonly suggest 6 guppies, but with the oversized filter in the fluval edge, you might be able to get away with 8. Just try to keep 2 females for each male if you decide to try to breed them.
The only Fluval filters I have seen and used are external cannister types and they are amongst the very best perfoming filters available. I have never heard of an in tank Fluval.
u can use a aqueon 55 gallon filter
yes you can it will be fine. i have experience :) haha
You may be able to use a very small internal filter with a 2.5 gallon tank. You can check with your local fish/pet store and they may be able to show you ones that would work for your tank
For a few days yes.
I would highly recommend it, yes
Clean filter
This depends on the strength of the filtration system on the tank. On a 'normal' five gallon tank (filter turnover about 5x/hour) three or four would be a good number. On a 'five gallon tank on steroids' (I have one of these - filter turnover 12x/hour) I kept seven with no problems. If you're going to keep more than three or four, regardless of your filtration, you need to be religious about tank maintenance - 50% water change, every week. Keeping more than 6 or 7 in the tank is probably not going to work. (For filter turnover, 5x/hour means the filter pumps 5 times the tank's volume each hour, so that would be a 25gph filter in a 5 gal tank.)
Depends which tank.
it gets 26 miles to a gallon tank
Yes. A goldfish can live without a filter. They may not live as long, but they do not have to have a filter, or a heater. I had goldfish in a five gallon tank without a filter for a long time. Hope that this helps.