You must get the same water I do... because I just tested mine today and the KH ~7 and the pH was 8.
One thing that is important is that consistency is more important than the actual measure. Tetras will do fine at a pH of 8, just as long as it doesn't change, everything should be good.
Now, about adding fish... I would reccomend getting something you really like. Look for a tetra that you like. Rummy noses, neons, cardinals, black neons, lemon, there are many more in addition. Get something you want. Also, I would reccomend getting a couple fish at first, then adding more. It is definitely safe that way, and fish lonliness really takes a toll in the long-term, and you would be adding more fish any way!
Adding filter media from your other tank will help a LOT. I did it to my 55 gallon tank and it was cycled in less than 2 weeks. Eventually, if you plant your 10 g more, the bacteria will actually die, since the plants will use the ammonia, and the bacteria won't get any, the plants out compete the bacteria. But this is fine, don't worry.
I would say a school of 4-6 tetras would be good with 2-3 ottos, but I would add 2-3 tetras first to help cycle the tank, however if you are picking a more delicate tetra to have, just cycle the tank with 2 zebra danios. Hope this helps! Good luck!