Heh. Funny how you love promix and I hate it
Its way too easy to over-water young plants with promix, thats even with 30% perlite added to it. It retains too much moisture for my taste.
OP, if you go with that brand, you're gonna need to add some perlite because its going to retain too much moisture, but Id rather get FF if using soil. Its a bit more expensive, but you can find it for 12-13$ for 1.5 cubic feet if you dont mind paying shipping. Maybe you're even lucky and live next to a place that sells it cheaply.