I want to get soft wheels that are able to excelerate fast, grip the ground pretty good, and are good for cruising. I skate with hard wheels size 51mm. What brand should I buy and how much millimeters should it be.
Would you put rims on a Kia or a turbo on a Neon? If your answer is yes, then I can't help you. Waste money and ruin the bike however you see fit.
I wouldn't go any smaller than 61mm for cruising wheels, and keep the durometer at about 75-90a. Bigger wheels will go faster, but take longer to accelerate. Softer wheels grip better and go over obstacles better, but are slower. Check out Pig, Santa Cruz, Sector 9, ABEC 11, Cadillac, Powell, Gravity Kryptonics, Bennett and Tunnel.