not to jump all over you, but there are several things to note:
1. goldfish are temperate (66-72 degrees), plecos, swordtails, and clown loaches are all tropical species (74-78). they should not be kept together.
2. what size tank do you have? fancy goldfish get big and fat, and will need a minimum of 30g for one individual. you can get away with 20-25g with lots of filtration and maintenance (water changes).
3. what type of pleco do you have? some stay small, but the plain old black or brown ones sold cheaply in most pet stores generally get very big, and are not suitable for most tanks.
4. clown loaches should ideally be kept in groups of at least 5. they like having friends, feel safer, and exhibit much more natural and playful behavior when in groups. clown loaches grow slowly, but they will get big, 12-15 inches eventually. they are easily stunted when kept in a small tank for too long.
on to your actual questions -
run an appropriate filter for the qt tank on your main tank (if you have a 10g qt, run a 10g filter on the big tank). then when you need to use the qt tank, just set it up and put the filter on it, then you don't have to worry about cycling it every time you use it.
do not add anything to the tank unless the fish show signs of sickness. some people like to treat for everything just in case, but i think it is not needed if you qt for long enough. after 3-4 weeks anything that would cause a problem would have shown up already. if the fish do get sick, wait 3-4 weeks after symptoms are gone. that is to make absolutely sure the fish are healthy. it also allows you to combat stress and condition the fish before they have to compete with other tankmates. temperature should be the same as the main tank, tropical for the loaches. you may want to put in a cave or two or some fake floating plants, which will give the fish places to hide and feel safer.
