[*]The Trev reg and large Ram I don't know the sizes of. The ram has push fit studs in but has no particular markings, just red and the number 8. The port beneath, by deduction from the cylinder bore, should be 1/4.

The bigger ram is a 40mm bore. The fittings in it have a 1/4 thread and will take an 8mm pipe. Obviously you can change these for whatever size pipe you need
[*]The buffer I was told is built with a 3/8 stud in it.

The fitting in the buffer tank is 3/8 thread and for an 8mm pipe. Again you can change this for whatever you want
[*]The sol valve has 3/8 ports with no studs.

Secondly, what metric pipe would I use for each port, for high flow rate?

The biggest easily available pipe using push fits is 12mm (you can get bigger though). We used to use 12mm on Hydra so it's probably overkill for a feather.

An 8mm pipe would probably be fine. I know you have a 3/8 valve but the reality is that this will not be fully open before the ram has finished firing. You can use the flipper calculator on the Hassocks Hog Page www.hassockshog.co.uk which should show you the difference between various pipe sizes.

Thirdly,How on earth do you refill the paintball bottle?

Take it to an event and get someone to show you how.