Fish Compatibility / Fish Disease

Question
Hello Jerry -

I am fairly new to keeping an aquarium - I've had mine set up about 3 months.  I did a great deal of research on how to properly set up the tank by reading 'A Simple Guide to Freshwater Aquariums' by David E. Boruchowitz.  I have a 35 gallon hex tank and got 5 red velvet swordtails fish about 3 months ago.  They have been doing fine.  Last weekend, following the directions of the book, I started adding fish (after cycling was complete and the water had stabilized - nitrate levels were safe, there were no nitrites, and the ammonia was at a safe level.  I purchased 2 emerald green cories last Saturday to add to the tank.  The book said that eventually I was to add about 6, but I wanted to make sure there was time for the water to stabilize before adding more.  
Last night, I noticed one of the cories spinning around in circles - they both have been acting strangely since I got them, so I didnt think much of it.  Tonight, as i was doing a water change, I noticed one of the cories was dead.
My questions are as follows:
1.  Do you have any idea what could have caused the death of this cory fish?
2.  Do you have any other combination ideas that I could use?  The combination the book told me was to add 3-5 swordtails, 6 cories, and 2 angelfish.  

Any help you can provide would be great.  Thank you.

Answer
Hi Katie, thanks for your question, I'll try to help.

First off let me say that going slow is the right way to do things but I think that 6 corys in a 35 hex tank is too many. There is not enough bottom area to support 6. I would aim at 3.

I have found all corys to be a bit of a pain to get acclimated right and even after 50 years in this hobby I lose them, often in the first week. I don't think they travel well.

As far as your stocking plan goes I have to disagree with it. Swordtails enjoy higher Ph and a little salt while Angels enjoy low Ph soft water. I, personally, would stick to the live bearers if you like them and maybe add Platys and Mollys.

If you really want the Angels I would try to trade back the Swordtails and just stock Angelfish in that tank.

Hope that helps and don't hesitate to ask a follow up question if I missed something.

Jerry