A couple of things...
1. We have not, and will not characterize HPE/MSM antennas on ArubaOS APs/IAPs, so there is no guarantee of optimal performance using non-Aruba antennas (in some regulatory bodies, using anything OTHER than Aruba antennas is forbidden and possibly illegal). So there's that...
2. The RP-SMA connectors on the 204 are diplexed to carry both bands. The J9719A is a 2.4Ghz only antenna so you lose the 5Ghz radio AND it appears to have N-connectors which is not supported on the 204. The J9171A LOOKS like our ANT-16 so it's hard to know for sure. I cannot find the connector type (i'm assuming it's RP-SMA since the 466 is). HP's page isn't pulling up the data sheet so I can't compare it to the ANT-16 and the J9171A antenna patterns.
To be safe, buy suported aruba antennas, but chances are the 9171A is the same antenna as our ANT-16, and if so (find me the data sheet and antenna pattern), you might be fine.