Plecos have been know to suck on goldfish, killing them.  So it's not a good idea to put them together in the same tank.

10 gallons is ok for a juvenile goldfish, but 20 gallons (or more) is recommended as a permanent house for 1 goldfish, because when full size they need room to turn around and swim.  And you don't want their growth to be stunted by the small volume of water, because it causes health problems and shortens their lifespan.

The 10 gallon tank probably looks big since this is your first fish, but when you start looking at other tanks, 10 gallons starts to look really small.

Remember to keep an eye on your water parameters so the tank stays cycled.

Good luck and I wish you and your fish a happy life  

Can apple snails survive in the same tank as a 4-5 inch Goldie? I heard they were avid snail killers so I've avoided it but I'd love to have something in there that eats algae.

It's kinda of iffy with goldfish and snails. It depends a lot on the temperament of each individual goldfish. Some people have had success with raising snails with goldfish while other do not.  Some goldfish will nip at the antenae and other body parts of the snail, which can eventually kill the snail.

I would suggest adding a mystry or trapdorr snail they get pretty big and the other snails will soon over run ur tank 8