Yes, you should try to change strings one at a time. Having said that, on most fiddles, the sound post will probably stay up if you take all the strings off, (If it falls over, get a bit of stiff wire with a sharp end, spear the post in the middle and put it back up.)

With regard to strings.

Prim are nice and loud and bright, Pirastro tonica are warmer but quieter. Martin Bluegrass take a month to settle and then sound pretty much like prim (not quite as good, but so much cheaper)

I don't like dominants, but I know plenty of people who do.

Hope that Helps.

Jon