how to adjust cold water pressure?

hello
were having our bathroom refitted and i have a few issues about what the plumber has done and would appreciate some advice.
the water pressure isn't balanced, the cold water in all taps now is very high. so everytime you turn it on its very noisey, water splurts out and when you turn it off it takes about 30seconds for it to completely close(i mean it drips abit for 30secs). i've questioned him about it and he said its because the fittings are new....he's newly qualified so i don't think he has the experience to know better (i didn't hire him so its abit frustrating). clearly the pressure needs to be adjusted but what/where is he supposed to adjust?
also, i've noticed on a few copper pipes that hes used plastic fittings as connectors....is this okay? as i assumed it has to be copper soldered together when joining pipes