What types of mechanical switches are appropriate for a 200 V DC 20 mA circuit
With switches reliability is an issue, if a switch stopped working then a product worth hundreds or thousands of dollars might not be able to turn it on all because of a switch. If you read switch datasheets they will most likely give a reliability number for actuation or lifetime. 

The problem is the current running through them affects the reliability, the more current there is, the more likely there will be problems with arcing and heating, both of which reduce the lifetime of the switch. (or if there is many times the rated current, it could heat the contact and melt materials or blow out the contact by vaporizing it!).

Switches are drawn in schematics but they don't usually show the resistance associated with the contacts (and other metal in the switch)

Typically the two worst enemies for switches are heating and arcing. AC is much easier on contacts because it extinguishes arcs (it crosses 0V many times a second) and because of RMS values (10Vpk AC has less power than 10V DC because it is not constant\sine wave). This means AC will heat less and have less problems with arcing VS DC. Less arcing and heating also means less wear on the contacts and longer lifetime.


  What are some other options I should consider?


Use the smaller switch to drive a larger relay if you need higher loads. In product's I've designed that are 1kW or more, I've used a smaller switch to drive a relay and then that turns the product on. 







simulate this circuit &ndash; Schematic created using CircuitLab


Source: Using a momentary push button as a latching on-off toggle switch

Typically the smaller switch is used to actuate something larger (like the circuits above). Typically a relay, SSR, or an electronic switch like a mosfet or BJT. Surprisingly enough, electronic switches are more reliable than mechanical ones because they don't have contacts that can break down (and oxidize). These days the latest transistors can switch hundreds of volts. 


  Or is it not a big problem to use a switch rated for 250 V AC in a DC
  circuit, especially if the number of times the switch will be actuated
  with an energized circuit is low (few dozen)?


The ratings are designed for lifetime and reliability for millions of actuations, the datasheet says if you keep the switch within the given ratings (voltage, current, temperature, humidity) it will last for tens or hundreds of thousands of actuations (50k for the D2S-10 and 200K for the D2S-01). If the switch only needs to last for hundreds of actuations, it could probably be derated a bit. 

If the application is for a lab or hobby project, the ratings could be considered differently than a certified product (RMA's and service calls are very expensive to companies). If your in a lab and the switch breaks, buy a new one, or get two when you order them in case you burn out the first one. 

 

Only the D2S-01 is technically rated (if the product needed to be tested at an ETL like UL) for DC and only 0.1A, Why is that? Because the switches were tested at those levels and ratings. Since most of the lifetime reduction in a switch is from either heating or arcing, these switches must be more susceptible to arcing from inrush current and opening the switch as it mentions that as being a leading factor in modeling reliability (pg 4 of the datasheet)

Looking at the datasheet for the D2S-10L13D shows that it has roughly 50mΩ of resistance, so I don't think heating is a leading factor as the D2S-5 and D2S-10 have  current ratings of at least 5A, at 50mΩ this equates to 5W to 1W of power (P=I^2*R), which the DC rating would only have 5mW of heating with it's rating of current. 



  Also, will two switches in series allow for double the switching
  voltage capacity?


No, because one switch will always close faster than the other one, and you'll still have roughly the same voltage across the other switch as it closes. The current will remain the same.