I can't really give you a definitive answer but I hope I can share some of the knowledge I have gathered in the past year, whilst learning how to, and making hot sauce.
Rejuvelac is lacto fermented. Which should mean that the knowledge I have acquired for making fermented hot sauce over the last half a year or so, should be broadly applicable.
First of all, on the subject of safe pH levels...
When making hot sauce in a commercial setting, where you have the equipment to properly make and test the sauce, commercial condiments such as hot sauce must have a pH of 4.6 or lower. I must emphasis that this is for professional manufactures who have expensive, calibrated pH meters. The reason I emphasis this is because the pH of 4.6 or less, is in order to prevent the growth of C. Botulinum. Which whilst rare, is really dangerous.
For home sauce makers, it's widely recommended to aim for a pH of 4.0 or less. Not only is this is in order to prevent C. Bot growing, along with various other food born nasties, that can make you seriously ill or even kill you. It's also to give a margin of error in the event that your pH meter isn't accurate or doesn't measure to two decimal places.
So your question of "possibly ok" and "definitely discard" ranges can partially be addressed by saying that if the pH of you Rejuvelac is above 4.6, then you can't know whether its safe. A pH of 4.0 though, and you're likely to be safe... but here comes the caveat. Is your pH meter correctly calibrated? You could test it against some newly purchased, quality pH test strips, or you could buy or prepare test solutions but short of doing that, a cheap pH meter should be used with the fact in mind that the results my not be accurate.
There are some steps you can take to reduce the risk of bad bacteria ruining your batch, both in terms of stuff that will make you ill or kill you, and in terms of just making something that tastes bad.
1. Clean.
First of all, thoroughly wash any equipment you'll use to make your fermentation, in hot soapy water. Bacteria hides in nooks, crannies, and gunk, and the name of the game is getting rid of any undesirable bacteria. So make sure your equipment doesn't have and residues or smudges of dirt.
2. Sterilize.
Step 1 was about getting rid of particles of dirt. Where as this step is about killing the bacteria on your equipment. The are various ways of doing this, including numerous products that you'll find easily from home brewing stores, and online. But the method I prefer for ease, cost, and availability is to soak all my equipment in a 2% bleach and water solution, for at least 20 minutes. Use "thin" unscented bleach, and double check it's concentration of Sodium hypochlorite, to make sure that you correctly calculate how much is required so that the solution you end up with is at least 2% Sodium hypochlorite by volume.
3. Sanitize.
Once you're finished with step 2, you can drain the bleach and water solution from your equipment and leave it to dry. Whilst it's drying though, it can be recontaminated by bacteria literally floating around or from you handling it. So it's also important to use a sanitizer. One of the most popular in the brewing world is called "ChemSan". It's an acid based, no rinse spray that you mix to the correct strength, apply to your equipment, and then leave for two minutes. At which point, you can pour off the excess if need be, and then use the equipment straight away. Being acid based though, you absolutely must not mix ChemSan with bleach!
The other thing you can do that will help create a safer product is to start with fresh, healthy produce, that's not dirty, but hasn't been washed to within an inch of it's life. The reason being is that the friendly lacto bacteria we need for fermentation, should already be all over and inside the produce itself.
You might not like the idea of the above process but I would recommend rigorous safety procedures because you're at a disadvantage when making Rejuvelac as apposed to fermented hot sauce or vegetables.
Most other fermented products, such hot sauce, are made using a brine solution, or by adding salt to the vegetable matter and creating a "mash". The correct amount of salt helps prevent undesirable bacteria from growing whilst Lactobacillus is given the opportunity to flourish and create a mono-culture. The Lactobacillus then lowers the pH of your product, as well as emitting carbon dioxide. Both of which help prevent the bad bacterias getting a foot hold.
Speaking of CO2... a lot of people will tell you that you can just cover your fermentation with a cloth, or that you should stir it or taste it daily but the CO2 created by the Lactobacillus forms a layer between the ferment and the air. Which helps stop bad bacteria growing, along with the various molds and yeasts. You want to do everything you can to minimize disturbing this blanket of CO2, and everything you can to protect it. Especially for longer term ferments. Which is why I'd recommend getting an airlock, or using a vessel with a tight fitting lid that you can burp regularly (clip top jars with a rubber band to hold the lid down works well), and keep the fermentation vessel closed as much as possible.
On the subject of stuff growing... in broad terms, have a look at Kahm Yeast. It's the one growth that isn't a big deal but everything else, especially dark, or brightly covered molds are usually a sign that you need to bin what ever it is you're fermenting... sometimes, if the growth is above the water line, you can wipe it out (I use a clean paper towel sprayed with ChemSan) but this will disturb the CO2 layer, and it will not guarantee that said growth won't just come back.
One last thought. Have a look at lacto fermented ginger beers for more information on safe, non-salty, fermentation.
