Cyclone III: Using PLL_CLKOUT (Clock Out) pin as general purpose IO?

I think, both questions are clearly answered in the device handbook respectively pinout tables.

The pinout table clarifies, that PLLx_CLKOUT is an optional function of particular regular IO pins. (The basic function is indicated in the column Pin Name/Function). So any of these pins can be used for regular IO, if not required as dedicated clock output.

Regarding pin capacitances, Table 1-9 in Cyclone III Device Handbook lists the standard and special pins capacitances. The said dedicated clock output pins are obviuosly belonging to the category I/O pin. This isn't surprizing because they are regular I/O pins. Their optional function doesn't involve changes to the I/O cell itself rather than input connectivity only.