Absolutely. I have a couple half-gallon jugs, as well as a half dozen gallon jugs. Fill the jug, add MC, then get your "shake weight" action on.
Once I get to where I'm using more, I'll mix it in a 5gal black bucket with a drill paddle. While you won't need to really worry about pH with sunshine/promix stuff, it isn't a bad idea to at least know what your initial water and final mix pH's are. My brother is using distilled water as his base, and after he's mixed up his is a bit low (5.5ish) for promix. Soil, along with promix (and sunshine, etc) should buffer your pH for you, but it isn't a bad idea to make sure it's close.
I ran into that problem, but mine was too high (like 8.4 after mixing it all up) so I hit up the store for the RO. Just because you shouldn't need to adjust it, doesn't also mean you should fly completely blind, either.
MegaCrop on its own is really very, very simple to use. Follow the general schedule, feed your plants, read the leaves. If they are light green, increase strength. If they are dark green, decrease strength. If they are that pretty, grass green.... bingo!
You may need to supplement calmag a little bit if you use RO water, are running good LED lights, or both. I am, and I do supplement with calmag. For soilless mixes, like what you're using, I use basic calmag (calimagic from gen hydro, or calmag+ from botanicare.) I use those because they have a little nitrogen in them. (1-0-0, and 2-0-0 respectively.) This works perfect because I also wanted to add a little more silica to the mix. As the silica (dyna grow's Protekt) is a 0-0-3 NPK, the nitrogen from the calmag offsets the potassium in the Protekt to keep the N:K ratio under 1:3 (1 part nitrogen to 3 parts potassium.) With the calmag+ and the protekt, that puts me well within the ratio.
The schedule from Farside that I posted above has that information on it, and for soilless mixes it's what I follow to the letter. The schedule is also great because even though the calmag was intended as the balance to the silica, it can work the other way too. If you need calmag, you still need to keep that balance. Thus, you can then add a little silica and you're in balance.
If someone needed/wanted just the calmag, but was worried about nitrogen and didn't want to add silica, True Plant Science makes an organic calmag that has no nitrogen. I use this in my RDWC setup and it's great.
(First batch I whipped up with tap for my 50gal RDWC took an 8oz bottle to pH down to keep it in hydro range. So yeah, no bueno. 
