Is it normal to use different backup yoyos for competition? Like using a KLR but once you get a knot then using your Majesty? Just want to see other people’s thoughts on this. 
That will work fine. I’d suggest having back ups of the same yoyo though.
For competition, I recommend that you use matching yoyos as much as possible. Even rotate through them while you rehearse so the condition of the bearing and response is as even as possible.
However, this is what I see at contests:
Sponsored players typically will have same make/model of yoyo.
Some of the better off(financially) players will do the same because they can.
A lot of the younger, especially non-sponsored players, bring whatever they have up on the stage. They’ll offen start with that top-dollar metal, but if something happens, they go to whatever is their next in line, which could be anything, including plastics. There’s no rules regarding this. Heck, if you don’t have a competition set of matching yoyos, just use your favorites.
I won’t ever compete, but if I did, I don’t think I want 4 Anglams on stage with me. I’d probably see about getting either Code 2’s, or if pressed, I might stock up on Octaves unless I can get more Freq. Wav’s
If I had to go full plastic, I’d probably go for a set of upgraded Classics.
I haven’t in the past but I usually make the top 15 when I compete. So no, it isn’t essential. However, I was finally able to get two summits in my hands and those are my mains now. So even though I don’t usually switch in a freestyle, it is very helpful to have a multiple. Basically having several different ones is ok, but you have to readjust to weight, feel, etc.