For engineering work, check the
National Rail and
FGW▸ websites.
A 20-minute change of trains at either Parkway or Newport should be absolutely no problem at all; in fact you'll probably end up being bored waiting ages for your connecting train, assuming the incoming service arrives at or close to right-time. In the worst case scenario at either station you'd have to go up some stairs and cross to another platform. Assuming you were coming up from
PLY» on an
XC▸ service, unless things were running out of course you'd most likely have a "same platform" connection at Bristol Parkway, so you'd just have to step down from the first train and wait in the same place for the second.
And trains defiently run on Good Friday?

I assume you have heard of online journey planners? Put in your journey details and provided you give the right date then they'll give you the right service. Yes of course trains run on Good Friday: there are only two days a year when the railway system shuts down completely and those are 25/26 Dec. It may be a modified service from a normal weekday but if you're brave and go
here then you'll get exact details.