What bulb should I use in 4" recessed lights?
The 953P does not come with a socket extension. I'm not sure where you got it or why. With an H99 I can't see the use of one.
You CANNOT use a 12v MR16 in an H99 can. In fact, the 1420 trim is not for use with the H99 series. It is ONLY for use with the H1499 series low voltage housings.
See here: http://www.cooperindustries.com/content/dam/public/lighting/products/documents/halo/spec_sheets/HR_ADV110509_990trim_spec.pdf
Yes, 50 watts is the incandescent wattage limit. The CFL or LED equivalent does not apply.