1: Is there any rule or guidelines to how deep one should cut (as long as the spindle is up for it)?
That is about it, don't stall the spindle, don't stall the steppers and don't break the tool. As a general rule stay less than about 1/2 cutter diameter deep when cutting full width but go deeper when the cutter is only partly engaged. And do half or a quarter of this with cutters less than 6mm.
2: Is there a way of calculating the correct feed and spindle-RPM?
Feed is based on the cut per tooth and the spindle rpm, spindle rpm is based on material being cut and the cutter.
With carbide cutters aluminum alloys can easily be cut at 1500 meters per minute which corresponds to a very fast spindle rpm on a small cutter so most likely the machine can be run at full speed. When High Speed Steel (HSS) cutters are used the rpm should be about 1/3 that used for carbide.
As mentioned feed depends on the cut per tooth; the distance the tool advances per revolution. It also depends on the depth of cut and the width of cutter engaged in the metal. At depth of 1/2 diameter and full width cutters above 10 mm can normally handle a cut per tooth of about 0.5% of the diameter; smaller cutters need to be scaled back toward 0.1%. When the full width of the cutter is not engaged in the metal the feed can be scaled up because the actual depth of cut per tooth is less than the amount the cutter advances per revolution. The rate at which the cutter advances is simply the feed per tooth multiplied by the number of teeth and the spindle rpm. For example 0.01 mm per tooth for a 2 flute cutter running at 20,000 rpm is 0.4 meters per minute.
3: When the flutes get packed, should I turn up or down the feed or the spindle RPM?
Yes, No and Maybe. Very often chips packing in the flutes is a result of no lubrication and/or too many flutes. Always use 2 or 3 flute cutters with a high helix angle for aluminum. Fewer flutes means more space for chips and the screw like action of the helix tends to pull the chips up out of the cut. Of course blowing or washing the chips away with air or coolant is always a good idea. Too much feed or too low rpm can enhance the tendency for chips to pack because the chip is thicker. Soft aluminum alloys, 'low temper' or annealed are terrible for packing.
4: Is there such a thing as too much cooling liquid (except for the mess)?
Not really but often simply spraying, or brushing coolant on the surface to be cut, is adequate. Coolant really becomes essential for deep, full width cuts running at the top end of the feed range for the cutter, rpm and material when cooling is as important as lubrication. To reduce the mess on an open machine the best is probably a mister that blows coolant and air to provide cooling, lubrication and remove chips. These can also create a coolant aerosol in the air so wear a suitable grade of breathing protection.
An open mind is a virtue...so long as all the common sense has not leaked out.