I hear ya...carbide is expensive!
Well, in general, the RPM is set by:
RPM = (12 * SFM)/(Pi * Diameter of tool).
I usually use [12 * SFM / 3 * Diameter] as a quick simplification.
I'm doing these calcs in our dang "American" units...but you get the idea.
There are guidelines for different materials being cut with HSS or Carbide.
There are many of these type charts out there...here is one:
Solid Carbide Speed & Feed
Carbide actually likes heat (to a point). Sometimes using liquid coolant on carbide actually shortens the tool life because if the coolant does not completely flood the flutes then the heat cycles (if that makes sense...hot outside the coolant flow, then cooled...hot...etc) - and the heat shock can lead to premature failure.
For a quick example...cutting 303 stainless. For a hobby machine a conservative feedrate might be a good place to start. Say as low as even 100 SFM.
Assuming 1/2" cutter.
RPM = 100*12/3*.5 = 800 RPM. Assuming 4 flutes at .0015"/flute = .006/rev = 4.8 IPM (~120 mm/min).
That is pretty conservative. A typical SFM for aluminum would be 10x that (1000 SFM) which would equate to 8,000 RPM and 48 IPM - most of us don't have that ability on these small mills. At work we run a 1/2 carbide with only 2 flutes in aluminum at about 10k and over 100IPM and it lasts forever....but we'll run a 4 flute 1/2 inch around 1600 RPM around 10-15 IPM in steel and it will be warn out in an hour or less.
The problem you may have with carbide on a hobby mill is that carbide is very hard - but very brittle whereas HSS isn't as hard but is more flexible.
Carbide can break surprisingly easy with a shock load, and hobby size machines aren't as rigid so they may vibrate more...generate more shock...break more carbide.
I wouldn't recommend it for interrupted cuts because of this fact either - as much as you can avoid it at least.
SO for the coolant part...I don't think it is NECESSARY...but chip evacuation is needed so a few air blasts would be nice.
The RPM restriction and rigidity of machines (and expense) is why a lot of people here on the zone will recommend against carbide...you just don't get the added benefit over HSS most of the time. I would at recommend at least getting some "cheap" HSS roughers and just finish with your carbide to make them last a bit longer.
Good luck with that steel!
PS - also be aware that a lot of the commercial cutter recommendations are based on an expected tool life of as low as 30 minutes...so take them with a grain of salt. They market to production shops and want people to buy more cutters of course!