Leave request app

Hello,

we have the following SAP ECC 6 HCM system:

• SAP_BASIS 731, SP-level: 0010
• SAP_HR, Release 604, SP-level: 0073

We're still running ESS/MSS on an old Java Portal.

My question:

Can we use an app for starting/approving leave requests?
Is it just possible via a Fiori App? Is that possible with our environment?
Then we need a Gateway server, right?

Currently we're building up a S/4 HANA system landscape (including Gateway server and CPI).
Could we also use that to run the Fiori Apps?

Thanks in advance!