Main sellers of 'Ray Wu', 'Paul Zhang' and 'Scott LED'. There is a few more but these are the one people who've been in this hobby know are trust worthy.
As to @tooms comment; 18AWG is best. The lower the number is, the thicker the wire is.
There is also advice about not using strip for it's structural integrity and not being easy to repair.
Also congratulations for an anniversary of 5 years, even though this is your first thread post .
I'm presuming now; You've joined ACL in 2015 to grab a 101 manual and have now come back. Coming back is a sign that it's your destiny to do this hobby!!
Is it better to order 2.5m lengths of strip for arches or if you are ordering custom, or specify a length with no pixels for the break between two arches?
Depends on the size of arches you want but my display uses 2.5m strip for arches, inside 2.6m of tubing so that the conduit feet attachments on the ends do not block the last LEDs. I got them ordered with the pigtails already connected from Ray from memory
Might be a little late in the year to get custom orders. Easy enough to add pigtails yourself if you order 5m rolls and cut to length. The break between arches will be a male and female pigtail on each end that can connect if I understand you correctly. Makes installation, storage and repairs much easier if each arch is independent.