It is difficult to cut aluminium with small bits.
The closest I have tried is a 1.5mm bit and I used 0.05mm cuts. I mostly use a 0.8kW chinese spindle, but the DC-spindle works as well for small bits.
18 gauge aluminium is something like 1mm thick I think. This means it will also be difficult to get it flat, the variation and tolerance of the plate can be more of a problem than the actual cut depth. Also one of the reasons the bits break often. I always start the cut 0.1 or 0.2 mm above the surface and add a bit of depth to all cuts to compensate, it takes a little longer but you are less likely to break the bit.
I flatten an extra wasteboard first and then clamp the plate down on it. It is easier if the plate is only slightly larger than needed for the part, but you might also get warping from cutting it out, so it will be a bit of a balance. Be careful with how much pressure you use on the clamps as well, it can warp when you start cutting out the part.
Another option is to glue (Loctite/cyanoacrylate) the aluminium to a flat piece of thicker aluminium (or MDF) with pressure to force it flat. You can use a heat gun carefully to release it afterwards. I went with the first option because it took less time and material.
Chattering, due to the plate not being stiff enough, can also be a problem but mostly for larger pieces. Not an issue when glued, but it is difficult to release large pieces that have been glued down.
You will probably need coolant/lubricant, compressed air doesn’t seem to work as well for the small bits. Either a cutting emulsion or WD-40/5-56/electronics spray work fine for me, but I prefer the emulsion as the sprays evaporate to quickly. The main issue seems to be chip clearance, use 1- or 2-flute bits if you can, they are not as sensitive. High grade bits made for aluminium also help, but they are typically very expensive.
The bed, at least on the 500mm X-Carve, flexes a bit, after I replaced it with aluminium extrusions this kind of work got a lot easier.