Ok, no problem using using something on hand already, I've done the same.
two different things used for different purposes.
Yes & no - you're replacing the analog mux on the arduino with an external mux and using the internal ADC..
MCP3008 replaces both the mux and ADC with another ADC (that is not muxed) and reads in the digital data from SPI vs the internal digital bus. But it does the conversion a lot faster.