I own an iPad mini (MD528LL/A, s/n F4LKQ4N7f193) I use mostly for music practice (iRig); can I upgrade to 9.2.1 and use the newer music apps?...I read where there were some issues due to the older devices!

There is nothing wrong with this update.

This update is designed to plug a long standing security hole that took Apple forever to plug.

Plus, this update was specifically tailored to address issues on older iDevice models.

My advice on doing ANY iOS upgrades/updates.

Ideally, your iDevice should maintain a minimum free storage amount of 4- 6 GBs or greater on any iDevice (the more free space, the better) , plus, more than 50%-60% battery status to ensure proper operation, as well as, to ensure the possibility of a successful iOS upgrade/update installation.
In addition it is best to upgrade an iDevice connected up to a computer running iTunes with you iDevice sync'd and backed up through iTunes rather than Over The Air (OTA) using a wireless WiFi connection, if this is possible.

Doing a major or critical OS upgrade/update can be risky to do over Wifi or cellular due to the potential of data interruption or data drop outs causing issues with the download of the OS data and the subsequent install to the device.

Also, it is better to do the upgrade/update through iTunes as you'll get the entire upgrade/update and not just a smaller sized WiFi download version/portion that is based on your model iDevice that may or may not download everything your particular iDevice needs.

If you have no other choice than to use Wifi, then make sure you have a strong and stable connection signal the will not induce the potential for data drop outs.

I prefer the iTunes method as there is minimal risk and I have never had ANY issues with ANY OS updates using this method.

Good Luck!