90EX for fill flash?? Eeeeyyuukk! You might as well just use your old iPhone because that's the kind of light you're creating with such a small on camera flash. Hope you like red eye, and lots of it!
I know you're looking for something small but at least get something that tilts and swivels. Consider the 320EX II. Now, I know it's not brilliant but I had it and it has some very useful features.
1. It tilts and swivels
2. It can act as a slave, and has group compatibility.
3. It has a zoom head (ummm kind of! Lol)
4. It's pocketable if you have medium sized jacket pockets. (It's shorter than a 430ex)
5. It works as a remote shutter release
6. It has an LED so you can find your keys in the dark
OK the LED is useless. Wish it had a AF assist beam instead. It does have that crazy rapid flash burst AF assist that is kinda garbage but works.
I had this flash for a while, it served me well as my intro into the flash world. I see you already have the 600exrt so you prob would only use this as a back up anyway.