wow im actually qualified to answer this. Lately at work ive been running test on different relays for life expectancy for use in a nuclear plant.

Relays can be latching or unlatching

they can be double throw double pole or multi pole.

-you will care about coil resistance(which you will need to limit the amount of current going thru them)

-maximum switching rate

-bounce

-you can get relays that work on 3v, 5v, 12v and upwards

-the contact metal

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At work we are using Relays from Kemet because they last 100E6 cycles at 15VA or 15W accross the contacts. Thats being tested at both 250v 60mA(~15W) or 15v 1A(15w). We cycle them at 10Hz.

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When switching capacitors you will need a break before make circuit which can be done in several different ways.