I would like to construct a 80-10M G5RV dipole...
Then I strongly recommend you review VK1OD's articles on the topic:http://vk1od.net/antenna/G5RV/http://vk1od.net/antenna/G5RV/optimising.htm
Whether such an antenna actually covers all the ham bands between
80m and 10m depends on the losses you are willing to accept and the
tuner you have available to use. For example, many hams use them
on 10m, even though the SWR is high and you may have 5 or 6dB of
loss in the coax (depending on type and length). That's 3/4 of your
power lost in the coax, and about 1 S-unit lower signals on both ends,
but you can still make lots of contacts when the band is open.
Can I just connect the 28' of ladder line to the coax...
That will shift the SWR curve somewhat. You can bring it back in for one
band by readjusting the wire lengths, but the original design is a carefully
crafted combination of lengths. If you have to, make up an extra 2' of
open wire line and stick that at the feedpoint before switching to the
ladder line. (It does also depend on the style of ladder line - if it has a
continuous insulation without windows, then the velocity factor might be
high enough to allow a 28' length to work.)
The primary cause of losses (at least above 80m) is the operation of the
coax at a high SWR. The shorter the coax, and the lower the loss per foot,
means a higher percentage of your transmitter power gets radiated.