I am willing to switch from 700x23c to 700x28c (front) and 700x25c (rear) I've just bought Orbea Onix TDI Road Bike that has 700x23c tires. They are very tough on the roads that I have around - I go to mountain highways and there's a lot of pavement cracks that feel too bad for me with the tires that I currently have. Plus, my bike is 8yrs old (in perfect condition though, barely used) and the tires are 7-8 years old, too. 
I'm looking to put:
- for rear tire, Vittoria Randonneur II City Tire - BLACK, 700 x 25
- for the front tire, Continental Ride Tour City/Trekking Bicycle Tire, 700x28
The reason I have to go with smaller on a rear is because 28mm won't fit there. I'd love to go 28mm on both.
My question, is this configuration will be ok for my road conditions.
P.S. I've been cycling for about 2 years and had a hybrid Trek bike, that I use now only for flat roads along the beach
I used a 28mm front and 25mm rear for years on a road bike where the seat stays were too tight for a 28mm on the rear.  I felt that it was the wrong way around (the "fat" tyre should be at the back) but it worked fine.  I never had any issues with control or skidding or the bike feeling unstable.

In my case I used Continental GP4000s in summer and Continental Gatorskin in (Australian) winter, but I used the same model tyre on the front and back - just different sizes.

You'll be fine.