well gee whiz i had a whole thing typed up and then my computer froze up. so ill begin again.

welcome to the board! you've come to the right place! as you now know, the rule is 1 goldfish per 10 gallons. becuase your goldies are small and its almost a 20 gal, they should be ok for the time being with a few extra water changes, until you can afford atleast a 20 gal (should be asap)
your filter is ok with the 15 gal, but its limits are 20 gal, so you could probably have a better one.
i definately would not go with one of those vacation food things that are blocks and sit in your water--the fish will eat as much of it as they can on the first day, so they would be down to nothing on the 3-4 day. i would look in to an automatic feeder if i were you, the have these little slots for each day you cant feed them, and you put however much food into them as you would feed them in a day. then it just drops that days food in when the time comes. i think they are around $25. of course, if you trust someone you know to feed them (and not too much) daily, you could hire a friend!
just a quick question- do you have a water testing kit? if not, you should look into buying one, they are very helpful when it comes to figuring out (if you have a sick fish) why they are sick. you should get one that tests ammonia, nitrites, nitrates, and ph.