In general yes you can run the PSOC at 3.3 V (all I/O will be 3.3), or you
can power specific ports at their own I/O level.
Didn't use the I2C buffers because i want to make a easy to adjust to other uC like
Kinetis or LPC, i don't know if those uC have one.
Yes, they in general have across their families I2C capability.
as always all the info are in the datasheets, in this case on the kit guide:
4.2.3 Power Supply System
The power supply system on this board is dependent on the source of the power. For most applications, you can use the 5 V supply from the USB connection to power the system. You can also connect an external power supply to the board for low-voltage applications. The kit supports the following connections:
■ 5 V from the KitProg USB
■ 5 V from the PSoC 5LP Target USB (this will not power the KitProg section of the board)
■ 3.3V to 5.5V from a regulated supply connected to VDD (this will not power the KitProg section of the board)
Note: In order to use an external power supply, while KitProg is connected to the PCB USB, remove diode, D1, from the board. This ensures that VTARG supply from KitProg is not supplied to the target device. KitProg measures the target voltage and adjusts the logic levels on the programming pins accordingly.
It is important to understand that this prototyping kit does not have any onboard ESD protection circuitry. Therefore, the power source for the PSoC 5LP Prototyping Kit must be of a high quality to ensure that the board is protected from any over-current conditions and swapped-power connections.
Thanks for the help
Glad to have been of help.