Unfortunately the police are not the best people to ask when wanting information about potential sentences.
Violent Disorder is a serious offence � the most serious of all the Public Order Offences � and carries a maximum sentence of five years. Although it is an �either way� offence (that is, it can be dealt with either in the magistrates� court or in the Crown Court) Magistrates Sentencing Guidelines suggest that cases should be committed to the Crown Court except:
�...in rare cases involving minor violence or threats of violence, leading to no or minor injury, with few people involved and no weapon or missiles in which a custodial sentence within the jurisdiction of the magistrates� court may be appropriate�
Magistrates' sentencing powers are restricted to six months so you can see from the above wording that a custodial sentence is almost inevitable, and is very likely to be in excess of six months. So the solicitor is (unsurprisingly) more likely to be correct than the police.
A guilty plea will certainly reduce the sentence, but it is unlikely to reduce it much below six months. The only glimmer of hope is that the solicitor may be able to negotiate with the prosecution to reduce the charge to a lesser offence (perhaps one of Affray) where the potential sentence is lighter. This may involve the guarantee of a guilty plea.
If not, I�m afraid a custodial sentence is almost inevitable.