When applying a metamagic feat to a spell, you must either use Metamagic Song to completely offset the increase in the spell's effective level or apply the metamagic feat normally and cast the spell at its higher level. You cannot partially increase the level of the spell (or "split" the cost) to reduce the number of uses of bardic music spent when using the Metamagic Song feat. Only one method can be used to pay for metamagic feats that affect a single spell. You cannot use the Metamagic Song feat to improve a spell with the Silent Spell metamagic feat. - emphasis mine
Does the emphasize section mean other metamagic reducers, ie Practical Metamagic can’t be applied? Or simply that you can’t partially reduce the spell by say only one level if you only have one bardic music use?
It only means that every time you decide to use a metamagic in a spell you need to pay the full cost of the metamagic like the example. For a Maximize Magic, you either pay for the 3 level increase OR cast the spell with a slot 3 levels higher. If you by chance have abilities that reduce the cost of a metamagic feat (or all of them) it is applied before you pay for the bardic music uses for it, reducing the uses spent.