Yes, the Mec-Gars fit the full size 75 CZs.
The SPO1 mags hold 19 like the Mec-Gar +2s do.
The only replacement spring for the 17 & 19 round Mec-Gars are the same size and strength as far as I know.
The only difference in the CZ and MG mag springs are the shape of the end loops of the MGs are shaped to fit the base plate. I'm told the CZ springs will work in the MGs.
The CZ springs can be purchased with "extra strength" +5%/+10%. The MG springs are not available with "extra strength".
Curious why you would want to change " back" to 17 rds with the +2 mags. They run faultlessly in the SPO1.