Yes, if they previously held ejuice, soap and water wash is fine. Heck, I don't even use soap, just lots of hot water

If you do use soap, DO rinse REALLY REALLY well!
No need to freeze the PG and VG, although keeping cool and dark is good. But you can, if you wish and have space. It won't hurt them. As is the case with the nic, let them come to room temp before opening, to avoid condensation in the bottles. And the VG, if coming from the fridge or freezer, may not even pour at those temps! Gotta warm it up a bit!
Your "beakers, flasks and syringes" could well be all the measuring stuff you need. Depending on the quantities you're mixing.
Like many, I tend to mix my 100 ml/mg nic down to about my desired strength. (Actually a bit higher, since the flavorings will bring it down a tad more.)
So, for you, say 6 ml of 100 mg nic, 69 ml of PG and 25 ml of VG would give you 100 ml of 6 mg nic in 75/25 PG / VG. But keep in mind that most flavorings are in PG, and, of course, have no nic. So going with, e.g. 7 ml nic, 64 ml PG and 30 ml VG might get you close enough to your desired final mix once those flavorings are added. You'll need to adjust that depending on how much (i.e. what %) flavoring you use. I'm rarely above 10% flavoring on my mixes, vape at 18 and like it around 70/30 PG/VG, and mixing my working stock to 20 mg nic at 65/35 works fine to keep me an ex-smoker
Do NOT mix a new flavor in large quantities! While certainly cheaper than ready made, no reason to be wasteful if it's something that you really don't like. Some mix 10 ml test batches, of new flavors. I tend to do that with new single flavors, so I have an idea of how they taste solo. So I have a clue on what to adjust when used in a blend

Once past that, I tend to mix new ones in 30 ml batches, since if I'm really determined, I'll wait a day before trying it, and still have enough to let it steep for a while to see if it gets better