Hi, little_mermaid.
Eagerly awaiting the promised pics of your beautiful new fish.
If I correctly recall, I believe the problem was that the filter had been cleaned and the bacteria killed, so I don't know if there would have been much point in retaining the media...
Not being an expert, I can only offer my own, meager experience for consideration.
While some may be lucky enough to have access to LFS's with strict quarantine procedures, over about the past year, virtually every fish, (apart from my first few male bettas,) that I've bought from a regular fish store has had some disease/parasite, unlike the ones I obtained from a basement seller and from whom I hope from now on to obtain more of my fish.
Yet prior to that, I have bought healthy fish from the same LFS to which I most often go...
Too many fish from too many places and conditions pass through the sales tanks of most LFS's for problems not to occur without that q period and any necessary treatment.
Personally, I'd never use filter media from any LFS, having had so many bad (and expensive, for one in my position,) problems often immediately become apparent of late - although I would from that basement seller...
little_mermaid, the advice given by people above is excellent - and the mondo grass will almost certainly eventually rot, producing ammonia which can both poison your fish and produce algae, depending on amounts involved.
I'd suggest keeping a close eye on it and throwing the lot away the instant any sign of die-back is evident, as it will likely live for a while.
(I know nothing myself, but a lot of people on the internet have shared their experiences of this.)
Congrats on the Cabomba, a lovely plant which has long been an aquarium standard in cool water tanks, and in some tropical as well, although on the other hand many, myself included, have discovered that it tends to disintegrate in tropical temps.
Doesn't mean yours will, and it does do quite well in some warm tanks...
The experts above have said it all, so don't ask me why I'm meandering...
But I'd really strongly suggest that you do a small daily water change at least for the first week, before going to every second day for a couple of weeks and maybe sticking at every 3 days for a few weeks, if you can't get mature media from a safe source.
Filter media cannot be rinsed in chlorinated water as you've learnt, because it kills the beneficial bacteria, as already explained above.
And an ammonia/nitrite peak can occur so rapidly and invisibly that fish can suffer permanent damage or even death prior to a problem being recognised.
In a tank the size of yours, this only requires a few days..
We have to bear in mind that fish even in the wild, and in isolated places, are suffering immune system damage due to industrial pollutants, and that fish generally can no longer tolerate stresses they once took in stride.
Healthy, fast-growing plants will help enormously in sucking up ammonia, but a certain proportion is required, and plants also need time to recover from the shock of transplant.
If you don't mind a couple of suggestions from the peanut gallery for plants you might want to watch out for, my personal favorite for smaller tanks is Cardamine Lyrata, a graceful plant which doesn't spread wildly, but which is, I suspect, a good ammonia sucker as it rapidly fades and dies away from ammonia-producing life.
It produces foliage through which fish can swim, and through which the tank can be viewed.
If you've never found this in a LFS, you might have better luck finding Pennywort, which is similar although larger-leaved and which will grow out to produce chained 'lilypads' on the surface even if planted on the bottom.
I believe the latter was previously recommended to you by (if I recall correctly) MrBliss, although I'm not sure of the availability of either in the average LFS.
Egeria densa is highly recomended as a hardy plant to help maintain good water condtions, but I (and others) found it typically dies if you dose Excel as a carbon supplement.
A tiny dash of Flourish after waterchanges makes the world of difference to plant health, and also adds minerals which may even also help fish health, or so I'd assume.
Don't know if any of this is of any use or help, but I did want to at least add a bit of support to you and to your new venture.
I'm sure all will go well this time.
Some areas just don't have quality shops. I'd definitely take ReMz's advice and see if someone will ship you media.
I went to the LPS to ask about the bio-balls and used filter media but they had neither; but while I was in line waiting to ask questions about all of this I couldn't help myself but look at all of the Bettas! And then I saw her! She was so different than the rest of the female Bettas! Her colors are so unique! So I bought her and some live plants for her new home...and so far she seems to be quite happy 
