Hi Lisa,
Your nice hard high-pH tap water is great for mbuna cichlids, good choice! One of the great advantages of doing a full fishless cycle (with a qualification week) is that your biofilter is then ready to handle a full tank stocking of fish. People almost never actually fully stock in one go because they usually have trouble finding all the right fish at once or some species they want need to wait for a more mature tank. If you're smart you'll try to do a pretty large stocking however, to take advantage of the only time you'll be able to do it. From then on, you have to only make small additions of two or three small fish or 1 larger fish and then wait a couple weeks to let the bacteria catch up.
Unfortunately, that same water that is what the cichlids like is not water that the angels will particularly like. They tend to flourish in softer, more acid water like that of the Amazon where they come from. Even more importantly, the smallest tank they should be in is a 30 US gallon tank, about twice the size of yours. Another important measurement of the tank is that it needs at least 17" of height.
One option would be to use your smaller tank as a quarantine tank for your large display tank. They are in fact needed (I too felt frustrated thinking of preparing a quarantine tank after I'd already prepared my main tank by fishless cycling!) What happens is that you quickly realize how much you'd hate to lose all those expensive and beautiful fish (and pets that you've grown attached to) just because a new fish brought in a simple ich parasite or some other disease. Unfortunately it is quite common for this to happen.
If it is too much to think of using your nice 15 for a Qtank then I would explore some of the other great possibilities in fish that like higher mineral content. At my LFS they've been having some really beautiful pearl white mollies (almost all mollies really like hard (high mineral content) water) and I could really picture a tank with a stark black and white theme, perhaps some large black rocks. Or a shoal of marble hatchets! Unusual!
The end of fishless cycling comes when both ammonia and nitrite(NO2) are dropping from 5ppm to zero ppm within 12 hours of when dosed and you've watched the filter perform this feat for 7 days in a row. If it doesn't blip during that week then it will be very unlikely to mini-cycle on you when you get fish.
To get fish you plan your big weekend by hopefully knowing that the fish are available. You then perform the big water change (down to the gravel to get most of the nitrate(NO3) out) and you obviously don't dose ammonia after that water change unless you don't intend to get fish for more than about 18 hours or something. There should be plenty of searchable threads about acclimating fish if you're worried about that aspect.
Once you have the big weekend, the clock begins ticking on normal tank maintenance. We usually advise beginners to start with weekly gravel-clean-water-changes and monthly filter rinses to begin with but then you can read up on how to optimize all of that. The weekly water change is an even more important secret to good tank keeping for beginners than the cycling is in some ways! Good luck!
~~waterdrop~~
