Your best bet is to find a good fish dealer to order through, there are several. If you can find one near your region shipping will be cheaper. Then you need to figure out how many tanks they are going to work with to determine how much you can stock.
Quaranting new arrivals is good practice, but honestly most pet stores don't. When you are getting them all from the same dealer, you are putting trust in their ability to keep their stock healthy before sending it.
As for what species, go with the easy stuff as you mentioned. Bread and butter fish like tetras, corys, etc, are not only cheap, they are popular. Once you get things rolling it might be cool to stock a few uncommon things, but don't go crazy with it. Your average customer is not going to understand or appreciate the difference between wild or f1 cichlid strain versus a tank of mixed mashups.
Not saying all people are like this, but many of your customers are going to want something cheap and nice to look at, so don't dedicate too much time specializing in the rare stuff until you get the basics covered.
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