If you tight mark and close down on all the opposition attackers it can lead to your defenders being pulled out of position so it is best (
IMO) to limit this to the oppositions playmaker or top striker or a dangerous winger.
Tackling set at hard will give the opposition a hard time but will get you a lot of yellow and red cards. Again (
IMO) it is best to allocate hard tackling to only one or two of your players ( I usually have my DM on hard tackling ) and usually against the playmaker who may lose interest unless he has a high bravery attribute.
Read the Unofficial Tactics Bible thread and the FAQ at the top of the Forum for further guidance.