Resolution is related to the output quality, but there's also a limit in the dot size of your laser. The 2500mW are typically 0.16~0.18, lower power lasers may be slightly smaller 0.12~0.18 and the higher power lasers more like 0.16~0.24.
So, let's assume your laser is focused at 0.16. You could pick a resolution of 0.15 for fine quality engraving (also slow), if the design is small and/or has a lot of detail this may be right, otherwise I normally de-focus slightly and use 0.2 for speed.
As you said, 0.2 would be 5 pixels per mm or 127 dots per inch. If you export an image at the size you want and select 127 DPI it will generate the correct pixels for the laser. If you use 254 DPI it would generate twice as many and be double the expected size, although you can and should set the output size in T2Laser to fix it.
So now you have resolution set, the feed rate depends greatly on the material. I would start at 800, if it's too dark increase to 1000, then 1200 up to 2500 or maybe a little higher and then if it's still too dark reduce the laser max power by 50. If it's too light just reduce the feed rate until it looks good.
As a reminder, if you are using my Grbl 1.1e (probably should be) then the laser min/max are 0 and 255.