You cannot use 40 Volume as it increases the alkalinity of the product and will make the product work unexpectedly. i.e. The color may heat up and providing erratic results as well as the possibility of damage/ breakage.
Changing the mixing ratio will affect the alkalinity. You will increase the alkalinity in a 1:1 ratio and as such increase the likelihood, you will get erratic results as well as the possibility of breakage.
Do not mix SoColor Cult with other Matrix Color. The technologies and mixing ratios are very different and would affect the outcome of the end-result.