Greeting from the Pacific NW,
Gerald here. Guessing you have the Flow-hive all assembled n ready to GO there. Glad you have the two Nucs (?) ordered .. Usually depending on when you ordered these should arrive mid to late April.
Curious about the 6 frames ?! I'm guessing the is a 6 frame deep hive body setup too ?! Odd amount of frames but what the heck !! Hives are hive .. I only use 5, 8, n 10 frame wide hive boxes. Not sure what supplier goes for a sixer wide. As long as you can buy extra 6 frame supers that should be good. You'll find the narrower hives (2,3,4, n 5's n maybe your 6 are usually called Nuc/starter colonies. Mine really grow upward like a chimney fire as the colony does have to draw comb widthwise .. Be ready with extra 6 frame wide supers. When you get your 5 frame Nucs that wood or cardboard hive is almost or totally ready for expansion.
You Flow-hive will have some growing space being several frames (8) wider but that 6 frame if you have local good bloom should almost have another 6 frame super ready to go on soon if not immediately or they will feel stuff n out of room thus get in the SWARM mood n that's not what you want to deal with so quickly.
I'd be finding a local bee club, some classes n a knowledgeable mentor to lean on at times. You'll have a lot less problems raising honeybees from the "Get-Go" !
Got more questions .. Give us a try here on the forum. There's a lot of experience n thots/ opinions here. But basic beekeeping is same same worldwide. You'll quickly gain experience n your way soon enough.
Good luck n happy beekeeping,