Yes most defenitley.
yes
canon by far
Seems as if the Speedlite 420ex can't keep up with the speed of the 50d. Many missed flashes.
Yes, you can. It has a Hot Shoe Mount.
Canon accessories does not work on all models.
Yes, but with some limitations. The Canon 480EG was designed as a TTL flash (through the lens metering) but also has an External Auto metering Mode. It can be used with any camera that has a PC sync connection. Don't use the "Shoe Cord E" that comes with the flash unless you are using a Film Camera with TTL flash metering. It can be used with Digital cameras provided you use a PC sync cord to connect the flash to the camera. Use the camera in manual mode and match the settings that you want to use with the flash.
That question is hard to answer, can't you just google it or read the 'prescription' haha?
If it's made for Canon, yes.
Any EF or EF-S lens will work with both the Canon 500D and 550D. So yes.
In literary terms, a canon is a highly respected body of work.
No
Yes. If the lens is a EF or EF-S lens, it should work fine with the t1i