default voltage for the cyclone 3 why is 2.5

If I understood you correctly, the answer is "yes".

Each bank has it's own VCCIO pins that power the I/O elements.

Within each bank, all I/O elements will be powered by the same VCCIO. But different banks can have different VCCIO.

Within each bank, the VCCIO must be compatible with the I/O standards used.

Ie, if you want to use 3.3V LVCMOS I/Os, the bank's VCCIO must be 3.3V.

OTOH, if you want to use LVDS signals, the I/O bank's VCCIO must be 2.5V.

Therefore, you can't mix 3.3V LVCMOs and LVDS signals in the same banks.

You need to use different banks.