Hi Invicta

My standard post-marian army would have:

10 Legionary Cohorts or 9 plus a First Cohort
2 Archer Auxilia
2 Legionary Cavalry
1 General.

The other 5 spaces would be filled with mercenaries/artillery as required.

Most forum users stay away from Praetorians/Urbans as being unrealistic.

I assume the advice you had been given was for a pre-marian army initially.