Visiting Houston from 27 Apr to 1 July

Hi, I'm from Asia and will be visiting Houston. I need a Nano Sim for my unlocked iPhone 5. Can I get the Nano Sim from any TMobile store in Houston?

I saw the website http://prepaid-phones.t-mobile.com/prepaid-plans and planned to do this way.

Get the prepaid $50/mth plan or $30/mth plan (still deciding) for 2 months and afterwhich change to the $2/day plan for the rest of the days in Houston. Can I do this way? How do I change from one plan to other plan? How do I refill the credit?

Other question is the unlimited web. Can I still continue to use the iOS apps (e.g skype, whatsapp, etc) to access the internet?

Thanks!