You can mix vermiculite and perlite. you don't press down perlite - you grow it as is. just wash it and take caution not to inhale its dust.
I always have heat problems... I try to run as much lights as possible and they never get me below 21°C.
pH of 8 is SLIGHTLY alkaline. its not the 6.8 marijuana likes but its not that far off. there are a lot of things to lower the medium pH and if your nutes don't do it you should do it yourself.