I don't use HSM, but I am using a number of Google Home Minis (and other Google Assistant enabled speakers) for a variety of announcements. So I think you could use the Google Home speakers via TTS, but I'm not sure you'd want to until the Chromecast Integration is out of beta, or at least more stable than it is now.
I've seen several threads suggesting that the current Chromecast Integration may be causing stability/performance issues with the hub. I haven't personally had any issues, but my setup is pretty simple.
The issue that I do have with the Chromecast Integration (Beta) is that it periodically stops working for no apparent reason, and that in itself would be enough of a reason for me not to use it as an alarm notification.
I use it to announce when a door is opened, which works most of the time. Occasionally, though, it will stop working and I won't get a notification when a door is open. Sending a Refresh command will usually play out the "stuck" notifications and get things going again, so I added a rule to send a Refresh to the cast group every hour. Well, actually now it's every hour except between 10PM and 7AM. I had an issue not long ago where it stopped working while I was out of town, and the refresh apparently didn't fix the issue until 3AM, at which point all of the Google Home speakers started playing out all of the announcements since it stopped working. My wife was not amused.
There are other options for using Google Assistant speakers, but I haven't really looked into them as the default integration is working well enough for me. The alternatives that I have seen (but not tested) are Assistant Relay, Cast-web-API, and VLCthing. Some (if not all) of those require a separate device as the gateway to the Google Assistant devices.
You can play MP3 files through GH speakers via Cast-Web-API but not via Assistant Relay. But I would not advise relying on this for security. If your RPi server were to crash, you would not get any alarm. If you're going to use HE security, I would advise investing in a siren device, preferably something with a battery backup so it works even if you lose power.