Hi LM and welcome to TFF!
I'm going to start with the following post quote as I feel it was particularly well written and probably the most important one up there, although the Assaye post is also essential and he's our point man on stocking and did an excellent analysis on yours.
A mature undergravel filter is the most effective biological filter that you can have, but not the easiest to care for. On the other hand, an immature UGF is as useless as any other biological filter is until it has the chance to mature. You have a heavily stocked tank but one that should be able to support its present population once the filter has a chance to mature. A trace of ammonia is an indication that the cycle on your filter may not have started yet. If you left behind filthy gravel, you may be introducing organics into the water, which will decompose into ammonia even if the gravel filter has matured. Either way, it will be important to stay on top of testing your water so that you will be able to tell when a water change is needed to control ammonia or nitrites. The relatively heavy fish population that you presently have will mean you need to change more like 25% to 30% of the water weekly, not merely 10%.
Our beginner section here has beginners/newcomers arriving in all sorts of different states of affairs and yours is certainly an interesting one, though it contains a lot of beginner mishaps that we see quite often. This thread contains such a large amount of information about gravel and rocks and especially fish that it has drawn our members, for the most part, into discussing all those things quite a lot!
I feel that there's the possibility that this reverses the priorities. First of all there's mention of the detection of white spot (ich) here and its not clear there's been proper follow-through on this either using OM47's excellent link in his signature or by posing up a query to Wilder over in the emergencies section. Maybe we think this is no longer a problem in your tank but I just thought we should check that out first.
Secondly, I think its pretty clear we've hardly started on the question of water quality and filtration. That is why I've quoted OM47's post. Its great that you've acquired a good liquid-reagent based test kit - that's really the first step.. and I hope that you've picked up a simple spiral notebook and are logging your daily test results (ammonia, nitrite(NO2), pH daily for now, nitrate(NO3) perhaps every 3 days or so.) Any other actions or observations should also go in to your little daily entry in the notebook.
One of the things we hope to accomplish in the beginners section is lots of learning, so that the experience extends beyond simply getting the first tank successfully started. It would be really good, if you haven't already done so, for the two of you to read the entries in the Beginners Resource Center on the Nitrogen Cycle, the Fishless Cycle and the Fish-In cycle. Those are really core concepts to the hobby and we don't want any beginner passing through to miss out on them. They also need to be well understood to help with the understanding of the filter situation and in your case its even more important since your filter situation is more complicated. UGFs, as OM47 stated, can be excellent (and inexpensive, which is one of their primary attributes) *biological* filters, but they can also be quite difficult for beginners for a number of reasons.
Certainly no more fish should be bought and the skill of good "Fish-In" cycling should be studied and worked over as a high priority here. The possibly mature gravel and UGF may turn out to be functioning well and that will be good but better to be safe than sorry in this area. The first learning goals should be the nitrogen topic (the nitrogen cycle and the different cycling processes) and then next the aquarium filter and what its different functions are and how they are achieved. This will be a lot to add on to the watching of an already going tank and the water changes it will be needing. The topics of fish and decorations (which are the wonderful goals that are of course wrapped up in the whole point of the hobby) should be viewed to some extent as distractions at this point, until the basics are confirmed to be well in hand.
~~waterdrop~~