That 'little' Dewalt compressor is made more for pneumatic nailers and such. Not really for 'shop' use. Skip it for use with the plasma IMHO.
The bigger 15 gallon compressor works OK with a PM30 for 'small' usage. I have just those pieces of equipment.

The compressor kicks on but the plasma keeps going just fine, at least for the small/short cuts I've made. Would I want such a set-up for all day long 'industrial' usage? Nope. But the compressor is adequate for small/short cuts with the PM30.
The compressor is also adequate to run an IR 1/2 inch impact wrench to remove lug nuts (air is being used in short bursts), but is NOT adequate to run a die grinder (more sustained air usage and the compressor can't keep up) or any other medium-to-high air demand sustained usage air tool. Believe it or not, a 3/8 inch air ratchet seems to use 'more' air than the 1/2 inch impact does because it runs slower RPM and for a longer period of time. The impact just 'zips' the lug nuts off, where the air ratchet is certainly faster than turning a socket by hand it is sure not as fast as that impact! Something to do with the 10,000 RPM free speed on the impact versus like 150 RPM on the ratchet.
The 15 gallon machine also works OK for filling tires too.
And although DesertRider's comment about the 'true' measure of compressors is the sustained airflow from the compressor pump -is- true as far as sustained usage, more air storage (bigger tank size -and- the pump sized to refill it in a reasonable time frame) does matter. Unless you have a big screw compressor and so much excess air capacity that it is being dumped back to atmosphere, a bigger tank gives you several advantages. It gives you a bigger 'reserve' of air for a tool that uses air in short duration but big 'gulps'. Like an impact wrench. IIRC, the actual airflow through a 'decent' 1/2 inch impact wrench during usage is something like 20-30 CFM. So if you are using an impact pretty much non-stop, you would need at least that much airflow capacity. But if you are just zipping off or on a fastener here and there, you can 'get by' with a much lower CFM compressor by having enough air tank capacity to do the zip-on or zip-off and then the tank refills (the pump comes on) while you move over to the next wheel or object, etc.
A tank is also important because it gives the pump and the compressed air a chance to cool off! Most air compressor pumps and motors are -not- made or rated to run non-stop (big industrial screw compressors not withstanding). They have a duty-cycle where they need to cool-off in between being used. The compressed air also should have a chance to cool off, so that the atmospheric moisture that was IN the air when it was compressed can have a chance to cool off and condense out of the compressed air before the air gets used in the tool/machine. Because when the compressed air expands back to atmospheric pressure in the tool/machine, it WILL cool off and then condense any moisture in the air back into liquid water. And air tools run much better on AIR and not globs of water mashing through them. Not to mention the rust/corrosion that water does to the inside of the tools. Or the mess that water does to say a paint job from a spray gun. Or how fast a plasma tip gets used up when water or wet/damp air is going through it instead of clean dry air.
And DesertRider's comment about "The more air the better" is sooooo true!
Small shop use, one person, intermittent use of air tools IMHO pretty much calls for 10-12 CFM as a -minimum-, and that might need to be higher if you get or want to use any higher air demand tools. Very limited use ( no high demand tools) or 'short' duration use (brief usage of an impact wrench for instance) and you can maybe get by with about 5 CFM and some combination of pressure/tank-size to give you 'burst' capabilty.
As Gayle mentioned, trying to run a tool or application (filling tires to 90 psi when the compressor only goes to 120 psi for instance) that needs a certain psi and a certain flow rate can be futile or frustrating unless you have enough 'overhead' for the compressor to drive the tool. Running a 90 psi app when the compressor only goes to 120 psi is tough, unless you have sooo much pump or a huge tank that the pressure never really drops down much even when the tool/app is sucking down the air. A bigger tank and/or more psi -in- the tank (which are the only two ways to get more air in the tank) give you that 'overhead'.
And skywest, although one -can- use a compressed gas cylinder to feed a plasma, it is usually not cheaper than a compressor. Certainly dry though! And quiet.
Let's say you have an 80 ft3 tank. At a flow rate of 5 CFM, that gives you about 16 minutes of use. Then your cylinder is empty. Then you have to swap out to the next cylinder. Another 16 minutes of use. And so on. So -each- 16 minutes of use would cost you a complete refill at about $30-$40 or so at the LWS. Plain air might be less, that's the approx cost around here for 'welding' gas like argon or nitrogen which is mostly the cost of handling to refill/swap the cylinders and not so much the cost of the gas itself. Not including the purchase cost of the cylinder in the first place. For $30 in electricity you could run that 15 gallon Dewalt compressor for about 66 hours of 'on' time at an electricity rate of $0.25 / kw-hr . That would be about 20,000 ft3 of air from the compressor in that time.
15 amps x 120 v = 1800 watts
$30 / $0.25/kw-hr = 120 kw-hr electricity used
120 kw-hr / 1.8 kw = 66.6667 hours of run time
66.6667 hr x 60 min/hr x 5 ft3/min = 20,000 ft3
Running the air compressor is waaaay less expensive than some compressed gas cylinders.
YMMV based on electricity rates and so forth, but you get the idea.
MHO? Get the biggest air compressor you can fit and afford. More air lets you do, well, more.

A 'decent' 60-80 gallon 3-5 HP (real motor HP, not marketing-speak HP. Grrr.) is usually about right for a small/home shop. Mobile use or space or electric power limitations may shift trade-offs around somewhat.
And a decent oil-lube air compressor is usually more durable and quieter than an oil-less air compressor. Just have to make sure the oil is there and clean (change it as recommended in the manual) and the compressor can't be run tipped on its side and run a decent air filter system to keep any oil out of things like the plasma or a spray gun.
And remember that one -can- often manifold multiple compressors as an air source. Not always the desired way to go compared to just having one 'decent' compressor, but sometimes a possible way to run things.