suitable tank mates for yellow lab cichlids

I don't know your tank size, but in my opinion and from my
experience, any of the peacocks make great tankmates. Also,
I have had success with a male electric blue ahli-Sciaenochromis
ahli. The intense blue coloration make a great contrast to the
yellow of the labido's. But I don't know of your connections to
acquire africans and also your tank limitations-size. But anything
with a bright blue makes for great color for your tank. Good luck!

You're really limiting yourself with such a small size, but you could probably throw some saulousi in there, maybe some demasoni, although they get pretty aggressive. If the tank is decent sized, you could mix up the lakes a bit, and throw some tangs in. Tropheus may actually work well, since there are alot of herbivorous tropheus and labs are pretty vegetarian too.

You could certainly add 2 or 3 Iodotropheus sprengerae. These are small mild mannered mbuna that only grow to about 3 inches. No risk of hybridization and a nice color contrast to the yellow.
http://malawicichlids.com/mw09000f.htm
http://www.cichlid-forum.com/articles/i_sprengerae.php