Can I replace just one front brake disc of my 2005 Honda Civic if the other side is ok and pads are good?

Yes , you can install just one disk/rotor without the other. BUT, the brake pads should ALWAYS be replaced on BOTH sides, AT THE SAME TIME. If you only replace pads on that one side with the new rotor, then you will run into the same problem later on, but on the other side. You want to keep your 2005 Honda Civic in the best condition possible. *** NOTE *** An improper working caliper is a major cause of a single pad wearing out prematurely compared to the rest. HERE IS A VIDEO. WATCH IT. https://www.youtube.com/watch?v=ar5j1mUdFA8 This video shows the ENTIRE caliper ASSEMBLY. The caliper (IN ORANGE) SLIDING on the 2 carriage pins (in light blue). These two "SEPARATE" parts need to be clean & lubricated to prevent sticking on one & other. If the caliper hangs up on one of the end slides , this causes a pad to wear on slant, eventually wearing quicker. It also puts most of the stopping force on other pad. That is the problem you need to address before just installing a new disk with new pads. ALSO, there's supposed to be a small metal tang on at least one pad, for each side. (High quality pads have them on both pads.) This is called a "wear indicator." It scrapes on the disk & makes a noise to alert the driver that the pads are bad. LOOK https://www.youtube.com/watch?v=2dFIbZ64w9c LISTEN https://www.youtube.com/watch?v=6yXFWUmi7FM 35 yrs experience here.

Not recommended , but sure , it's your car (I presume). BEST would be to find out why the brakes wore the way they did. Was it the left front that wore out the inner pad ? From what I've seen , that symptom is most closely related to driving habits/conditions. I would also check the RIGHT rear.........That would verify my thought/opinion. ALSO , don't mix brands of parts! REPLACE with the same as on other side.