yes, perlite is the white styrofoam in soil, but it seems to me that pot plants like a bit more perlite in the mix than what comes premixed in most soils. The perlite allows for better drainage of water, and more air pockets in the soil. trust me, the roots of your plants will love it...
As for fertilizing, (again im no expert) I started to slowly introduce nutes (using mg soil) when my plants had 2 - 3 sets of true fan leafs. I only used about 1/4 of what was recommended and increased from there. the only nute problem i had was when i started to induce flowering, and my dumbass switched to a flowering nute with an npk of 10-54-10, not realising that the plant would actually do a lot of VEGETATIVE growth (stretching phase) during the 1st week or so that my lights were on the 12 - 12 light cycle. in doing this i burned my plants pretty bad due to the high amounts of P nutes when the plant still really needed mostly N. I think i also may have given them too much of this solution for the first feeding of flowering nutes. anyway, here are what my plants looked like about a week before i switched to flowering.