soil question on first grow/ PICS

hey, i'm about a week and a half into my first grow, and i have four healthy (as far as i can tell) plants. I'm gonna buy some soil to transplant them this next week, and i'd like to get fox farm ocean forest, but i really can't afford more than one bag (1.5 cubic feet). i have some vermiculite and miracle grow moisture control soil. is there something i can mix the one bag of fox farm with to have enough for 4 plants (going into 3-5 gallon pots... haven't decided) or do i need more. advice please? here are some pics to give you an idea

im using ocean forrest my self ... i understand that u can only afford one bag but.. i wouldnt mix that soil it has a balance PH... dont mix it with miracle grow tho ..hell no....

how much does 1.5 cubic feet actually fill up? two gallons?

i would fill it up with 4-6 inches to spare .... it should be enough for 4 pots

sorry to bomb in on this thread but can you keep a plant in one pot for its life?
i have mine in a reaspnbly sized pot thats half filled qith soil... now if i just add more soil on top and cover up the stem if you will... will the roots grow upwards and take advantage of the new soil or would it be better to 'repot' with diff soil in there but a bit more so its to the top?