Hello and welcome.
Will the Gateron switches fit on the Iris PCB?
Gateron KS-3 are Cherry MX clones so any board made for the latter will work with your switches. Although know that the Iris PCB comes in two version and one of them is made exclusively for the
Kailh PG1350 series which are low profile and have a different pins position than MX-like switches. In short, double-check what PCB you're buying.
Where can I buy Gateron White switches?
They are actually clear (or translucent), rather than white. If you can wait weeks to deliver, you can rely on
Aliexpress.
do I need to buy PCB-mounted or plate-mounted switches?
The PCB supports both and, if you're using a plate, either will work. On the contrary, if you're not buying a plate, you should use PCB mounted (5 pins) switches.
I have no experience with Plovers or flat keycaps so I'm leaving those questions to more experienced gentlemen.