My coolant recommendation would be to use a "full synthetic," or other totally bio-hard coolant, to avoid ever having the sump go foul. You really can't have your coolant go foul in your situation. It's a tedious multi-step process to get the sump clean again, and it's probably just going to foul faster the next time. Sour or rotten smelling coolant will probably also scare students away from the industry.
You need to use RO water in your sumps or the minerals will build up over time and cause problems. These systems are not too expensive, and a common household four or five stage "under the sink" triple filter + RO system will suffice, although the bladder tank on those is usually only a couple of gallons, so building a system with a 14 or 20 gallon storage tank would allow you to fill multiple 5 gallon buckets back to back without waiting on the system, which is very slow to refill once drained. There are RO membranes with a 1:1 reject ratio, and those waste less water. There are systems with disposable twist on filters which require less care to swap filters when necessary, although you can go years without swapping filters on a shop RO system.
I used Syntilo 9918 (full synthetic) for many years and it is good as far as operator tolerability and unlimited sump life, but it is not the most lubricative for tapping, though the same might be said about other full synthetics. I'd usually add an M0 to my programs and apply straight coolant concentrate to the holes as a tap lube, which works fine and doesn't contaminate the sump. The rust prevention of 9918 was not perfect but probably on par with most other coolants.
Vises should be removed from the machine table every so often and cleaned underneath, if they are normally left on the table in your situation, maybe every month or two, and if brass / copper / aluminum are machined at the same time you really have to clean everything thoroughly after each project or job, or serious corrosion will occur. Plugging the center hole in the base of Kurt vises will help keep chips from packing under there and causing corrosion.
The sumps should be pumped out and the tanks cleaned occasionally, once per year maybe, or more frequently if brass / copper / aluminum are all machined in the same machine, but the same coolant can be reused to refill the tanks. Pumping the coolant into a 55 gallon drum and storing it there over the summer, if the machines go unused during that time, would prevent evaporation of the water and keep the humidity lower in the shop during the break.
I recommend Hangsterfer's 5080; it's what I use for medical device prototype parts. I mix it with distilled water. Maintain the proper concentration and it doesn't get stinky, no rust problems, works great in all kinds of materials, and it's basically food grade (emulsified soybean oil) so I can just flush it down the drain if I want to drain the sump, which I'd recommend over the summers, though you might get away with not doing so. I've even splashed it in my eye (not intentionally), and it didn't even sting. You can get it by the five gallon bucket if a 55 gallon drum is too much.
I took your advice and swapped my mill to Hangsterfer's 5080 just last week.... So far I am not impressed. This stuff smells strong and hits me like mustard gas. I cannot breath at all when I open the doors to my mill or it will choke me out, and this is without any "fog" being generated in my machining processes. The vapor just burns to breath, it's a dreadful sensation. Granted I mixed it a too heavy, 14% at first, then diluted to 11%. I will hope for the best as I dilute it down to 6-7%. If I get it to 6-7% and it is still intolerable I'm going to follow you around this forum every time you recommend the stuff and un-recommend it.