Based on listening to all the good advise on this site, I realized I had way to many fish and way to small of a filter. My tank is 120 gals US. I just got a FX5. I have narrowed down to 1-6in Oscar, 2-4in JDs,1 jewel, and 2 small Salvina. Does this amount of fish seem correct? Provided I don`t overfeed, should I still expect to do 30% water change weekly?

I would say you have a nice happy balance. As for water changes, it's difficult to say exactly how much water you will need to change on a weekly basis, you'll just have to do carry out nitrate tests and that will give you a good indication. However, weekly water changes are a good idea anyway, so it may be that you have to do a fairly large water change, or maybe just a small one to keep nitrate levels down.

Your stocking levels are good. As OFL says do a weekly water change. Test before doing the water change and if nitrate is above 40 then you will need to do bigger water changes. This is assuming you are not overfeeding (only feed what they will eat in 2 mins twice a day if small, once a day when they get to 4inches long)

I like to do a 30% water change every week. But if I find my test results are not on the mark I will do one at that time.

doing a 30% water change even without problem yet is a good habitual ritual
with the tank and fishes. you can't get wrong with that and a sure good water
maintenance. however, for the sake to know how long it will take before your
stock could pollute your tank, i suggest you let the tank settle for a while
and keep the testing so that you will know.

with my present stock (2 x 24" ID shark; 3 x 10" oscars; and 1 x 12"aro), and
my present filtering system, i am comfortable within 8 weeks without water change.
however, having a bonding with my fishes, i will do a few gall replenishment
whenever i do the washing of the pre-filter. that will add more comfort zone
for me.