If you use a compatible Power Adapter with your iPad, you needn't over-think the charging process - as the iPad will automatically regulate and optimally charge your iPad.
A compatible high-power USB PD Power Adapter is completely safe to use with your iPad; providing that the Power Adapter itself is healthy, it will not cause any issues whatsoever. That said, if a fault were to occur, the a high-power Adapter has the capacity to cause considerable damage to low-power electronics.
Apple’s current range of Power Adapter’s, including those intended for Mac computers, support USB PD (Power Delivery). USB PD is an active protocol; the source (Power Adapter) and sink/load (in this case, your iPad Air) “negotiate” and agree a common Power Profile that will determine the charging voltage that will be applied.
For non-PD compliant devices - or compliant devices that fail to negotiate a mutually supported Power Profile - the USB PD Power Adapter will only supply the standard USB 5V at a reduced current. This mechanism ensures that the higher charging voltages not supported by non-PD devices are not damaged by an over-voltage condition.
You should note that while your iPad Pro can be charged with a 45W (or other) Power Adapter, it will not charge significantly faster than when using the USB-C 20W Power Adapter that was likely supplied with your iPad. When charged with a USB-PD Power Adapter, the maximum iPad charging rate is limited to ~20W (although some newer model iPads reportedly accept up to ~28W). When using a standard USB Power Adapter, operating at 5V, the maximum charging rate is limited by its internal charge controller to 12W (i.e., 2.4A @5V).
Where the higher-power Power Adapter does provide benefit is when being used with other USB devices via a USB hub. A higher-output Power Adapter provides a greater “power budget” - allowing other devices to receive power while maintaining the iPad’s maximum 20W charging rate. For example, a USB multiport hub may typically require 10W power; connected USB devices (external storage, keyboard, mouse etc) also draw additional power from the source. If the connected load exceeds the available supply, the iPad may not charge - or may charge at a substantially slower rate. In extremis, both the iPad and Power Adapter will share the connected load - and the iPad may actually discharge its battery to make-up the shortfall in available power from a connected Power Adapter.
You should note that while Apple devices generally support USB PD, they do not support other charging standards - such as QC (Quick Charge).
