SAP PI is available for NW 7.4 both Java (AEX) or Double Stack. Support for Double Stack PI is an exception to the rule to support only one stack (either ABAP or Java) in the future NW releases.
PO is pe se an Java-Only-Stack-Release.
So technically there's no restriction to use PI with NW 7.4 (but check database dependencies). If you are unsure on whether to chose PO over PI you may want to consider your business needs and your current licencing model.
References:
SAP NetWeaver Discontinues Dual Stack
Re: PI 7.3 Dual Stack Vs Single Stack
Please come back if you have more detailed questions.
Cheers
Jens